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Basis Sets Ryan P. A. Bettens Department of Chemistry National University of Singapore

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We’ll look at… what are basis sets. what are basis sets. why we use basis sets. why we use basis sets. how we use basis sets. how we use basis sets. the physical meaning of basis sets. the physical meaning of basis sets. basis set notation. basis set notation. the quality of basis sets. the quality of basis sets.

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What are basis sets? Simply put, a basis set is a collection (set) of mathematical functions used to help solve the Schrödinger equation. Simply put, a basis set is a collection (set) of mathematical functions used to help solve the Schrödinger equation. Each function is centered (has its origin) at some point in our molecule Each function is centered (has its origin) at some point in our molecule Usually, but not always, the nuclei are used. Usually, but not always, the nuclei are used. Each function is a function of the x,y,z coordinates of an electron. Each function is a function of the x,y,z coordinates of an electron.

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An Analogy We desire to reproduce this function |x 0 >,|x 2 > as basis functions (BF) |x 0 >, |x 2 >, |x 4 >, |x 6 > as BF|x 0 >, |x 2 >, |x 4 >, |x 6 >, |x 8 >, |x 10 > as BF |x 0 >, |x 2 >, |x 4 >, |x 6 >, |x 8 >, |x 10 >, |x 12 >, |x 14 > as BF c 0 = 0.99651 ± 0.000907 c 2 = -0.16358 ± 0.000302 c 4 = 0.0078816 ± 2.68e-05 c 6 = -0.00017293 ± 9.74e-07 c 8 = 2.0326e-06 ± 1.75e-08 c 10 = -1.3396e-08 ± 1.64e-10 c 12 = 4.6961e-11 ± 7.68e-13 c 14 = -6.8392e-14 ± 1.42e-15

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Why use basis sets? We desire one or both of the following. We desire one or both of the following. The electronic energy of our molecule. The electronic energy of our molecule. The wavefunction for our molecule so that we may calculate other properties of our molecule. E.g., dipole moment, polarizability, electron density, spin density, chemical shifts, etc. The wavefunction for our molecule so that we may calculate other properties of our molecule. E.g., dipole moment, polarizability, electron density, spin density, chemical shifts, etc. We satisfy our desire by solving the stationary state Schrödinger equation. We satisfy our desire by solving the stationary state Schrödinger equation.

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Solving the Stationary State Schrödinger Equation (1) We wish to solve: Ĥ = E We wish to solve: Ĥ = E Ĥ is the Hamiltonian operator. Ĥ is the Hamiltonian operator. is the wavefunction. is the wavefunction. Ĥ is nothing more than a mathematical recipe of operations to be applied to the function such that we obtain a constant times back again after performing the prescribed operations. Ĥ is nothing more than a mathematical recipe of operations to be applied to the function such that we obtain a constant times back again after performing the prescribed operations. The constant will be the energy. The constant will be the energy. In the Schrödinger equation, the only thing we know before hand is the formula for Ĥ. In the Schrödinger equation, the only thing we know before hand is the formula for Ĥ. The formula for Ĥ involves operations that apply only to the positions (coordinates) of electrons and nuclei in our molecule. The formula for Ĥ involves operations that apply only to the positions (coordinates) of electrons and nuclei in our molecule.

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Introducing basis sets… In order to met our earlier desires we must figure out what (the wavefunction) is and with that we will know E. In order to met our earlier desires we must figure out what (the wavefunction) is and with that we will know E. Unfortunately we can only solve the Schrödinger equation to obtain nice formulae for when we have an hydrogenic atom (H, He +, Li 2+, Be 3+, …) Unfortunately we can only solve the Schrödinger equation to obtain nice formulae for when we have an hydrogenic atom (H, He +, Li 2+, Be 3+, …) If we desire to solve the Schrödinger equation for any system with more than two particles (a nucleus and an electron) then we are forced to make guesses as to what is. If we desire to solve the Schrödinger equation for any system with more than two particles (a nucleus and an electron) then we are forced to make guesses as to what is. One guess is to use functions that are similar to the formulae obtained already. One guess is to use functions that are similar to the formulae obtained already. That is, functions like s, p, d, f etc. atomic orbitals (AO’s). That is, functions like s, p, d, f etc. atomic orbitals (AO’s). At this point we might call basis sets, very loosely, as sets of functions like s, p, d, f, etc. that will be used to describe the behavior of electrons in all systems whether they be hydrogenic or not. At this point we might call basis sets, very loosely, as sets of functions like s, p, d, f, etc. that will be used to describe the behavior of electrons in all systems whether they be hydrogenic or not.

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Solving the Stationary State Schrödinger Equation (2) Ĥ KnownGuess = E Only if actually is the wavefunction E E E E If is not the wavefunction otherwise = Something else

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Approximately Solving the Stationary State Schrödinger Equation (1) Ĥ = E Ĥ = E 2 ∫ Ĥ d = E ∫ 2 d E = ∫ Ĥ d / ∫ 2 d If instead we approximate by then we can show that If instead we approximate by then we can show that = ∫ Ĥ d / ∫ 2 d We can always find an energy, , this way. We can always find an energy, , this way. A theorem in QM states that the ≥ E. A theorem in QM states that the ≥ E. If ≈ , then ≈ E. If ≈ , then ≈ E.

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How we use basis sets Basis sets are used to approximate . Basis sets are used to approximate . The bigger and better the basis set the closer we get to , and hence E. The bigger and better the basis set the closer we get to , and hence E. Nowadays almost everyone utilizes gaussian functions in basis sets. Nowadays almost everyone utilizes gaussian functions in basis sets. One or more gaussian-type functions are used for each AO in each atom in the molecule of interest. One or more gaussian-type functions are used for each AO in each atom in the molecule of interest. Let’s look at an example – the H atom, for which we already know what should be. Let’s look at an example – the H atom, for which we already know what should be.

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Case Study: H atom (1) We know that when we solve the Schrödinger equation for the H atom we get as possible wavefunctions: We know that when we solve the Schrödinger equation for the H atom we get as possible wavefunctions: = 1s, 2s, 3s, 4s, etc., as well as the p and d… functions etc. = 1s, 2s, 3s, 4s, etc., as well as the p and d… functions etc. The lowest energy state is = |1s>, with E = -½ a.u. The lowest energy state is = |1s>, with E = -½ a.u. The first excited state is = |2s> The first excited state is = |2s> Mathematically these functions (in a.u.) look like: Mathematically these functions (in a.u.) look like:

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Case Study: H atom (2) Graphically the 1s and 2s orbitals look like. Graphically the 1s and 2s orbitals look like.

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s Basis Functions Note that the exact s functions are of the form e - r (i.e., a Slater), where is a constant ( = 1 for H’s 1s, = ½ for H’s 2s). Note that the exact s functions are of the form e - r (i.e., a Slater), where is a constant ( = 1 for H’s 1s, = ½ for H’s 2s). Gaussian basis functions don’t even have the same form. Gaussian basis functions don’t even have the same form. s basis functions (g s ) are take the form: s basis functions (g s ) are take the form: Note Where is again a constant. Not

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Contracted Gaussians Sometimes a single gaussian function (a single gaussian is termed a primitive gaussian) can be improved upon. Sometimes a single gaussian function (a single gaussian is termed a primitive gaussian) can be improved upon. A basis function can, in general, be written as a linear combination of primitive gaussians. A basis function can, in general, be written as a linear combination of primitive gaussians. Here N is termed the degree of contraction. Here N is termed the degree of contraction. The d are simple numbers called contraction coefficients – they are fixed for the basis set, and do not vary in any calculation. The d are simple numbers called contraction coefficients – they are fixed for the basis set, and do not vary in any calculation. The g are the primitive gaussians, and could be s, p, d, f, etc. type gaussian functions. The g are the primitive gaussians, and could be s, p, d, f, etc. type gaussian functions.

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Minimal Basis Sets Minimal basis sets are constructed such that there in only one function per core and valence AO. Minimal basis sets are constructed such that there in only one function per core and valence AO. For the H and He atoms, we only have one function, because H and He have no core AO’s and there is only one valence AO – the 1s AO. For the H and He atoms, we only have one function, because H and He have no core AO’s and there is only one valence AO – the 1s AO. For Li – Ne the electrons in each element will have their behavior represented by 5 functions For Li – Ne the electrons in each element will have their behavior represented by 5 functions 1 function allowing for the electrons in the 1s core AO. 1 function allowing for the electrons in the 1s core AO. 4 functions for the electrons in the n=2 valence shell, i.e., 2s (1 function) and the three 2p (3 functions) AO’s. 4 functions for the electrons in the n=2 valence shell, i.e., 2s (1 function) and the three 2p (3 functions) AO’s.

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Minimal Basis Set Notation A minimal basis set is often represented by the notation STO-nG, where n is some non-zero positive integer. A minimal basis set is often represented by the notation STO-nG, where n is some non-zero positive integer. STO stands for “Slater Type Orbital”, with n primitive gaussians (the “G” above) will be used to approximate it. STO stands for “Slater Type Orbital”, with n primitive gaussians (the “G” above) will be used to approximate it. n actually specifies the degree of contraction that will be used to approximate the STO. n actually specifies the degree of contraction that will be used to approximate the STO. n is often set to 3, thus a STO-3G basis set is common. n is often set to 3, thus a STO-3G basis set is common. Minimal basis sets represent the simplest (almost the cheapest and nastiest – there is something else worse!) approximation we can make when we evaluate . Minimal basis sets represent the simplest (almost the cheapest and nastiest – there is something else worse!) approximation we can make when we evaluate . To make all this clearer let’s go back to the H atom case study. To make all this clearer let’s go back to the H atom case study.

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STO-3G N = 3. N = 3. For the H atom we have the following fixed constants that will be used to define the one and only one basis function H possesses with the STO-3G basis set. For the H atom we have the following fixed constants that will be used to define the one and only one basis function H possesses with the STO-3G basis set. 1 = d 11 g 11 + d 12 g 12 + d 13 g 13 1 = d 11 g 11 + d 12 g 12 + d 13 g 13

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STO-3G for H (1) These three primitives add together to give the contracted basis function Largest Smallest

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STO-3G for H (2) There is only 1 basis function for H. There is only 1 basis function for H. No flexibility at all in computing = -0.4665819 a.u. No flexibility at all in computing = -0.4665819 a.u.

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Introducing “Molecular” Orbitals By analogy with LCAOMO, modern QC calculations construct MO’s via basis functions. By analogy with LCAOMO, modern QC calculations construct MO’s via basis functions. i is called an MO, even if the calculation is applied to an atom, in which case they are in actual fact AO’s. i is called an MO, even if the calculation is applied to an atom, in which case they are in actual fact AO’s. c i is called an MO coefficient for MO i, even thought the coefficient is applied to basis function c i is called an MO coefficient for MO i, even thought the coefficient is applied to basis function

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Approximately Solving the Stationary State Schrödinger Equation (2) Recall that we desire to solve, = ∫ Ĥ d / ∫ 2 d Recall that we desire to solve, = ∫ Ĥ d / ∫ 2 d We want the lowest possible, because our ≥ E. We want the lowest possible, because our ≥ E. The MO’s are contained within our function. The MO’s are contained within our function. The only variables we have that we can change in order to get as low an energy as possible is the MO coefficients, i.e., the c i. The only variables we have that we can change in order to get as low an energy as possible is the MO coefficients, i.e., the c i. So all the c i are varied iteratively to minimize the . So all the c i are varied iteratively to minimize the .

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STO-3G for H (3) For our STO-3G on the H atom, we had no c i, so nothing could be varied here to obtain the lowest possible. For our STO-3G on the H atom, we had no c i, so nothing could be varied here to obtain the lowest possible. The of the H atom with a STO-3G basis set is thus completely fixed at = -0.4665819 a.u. = 12.697 eV. The of the H atom with a STO-3G basis set is thus completely fixed at = -0.4665819 a.u. = 12.697 eV. Compare with the exact result of 13.606 eV. Compare with the exact result of 13.606 eV. This is an error of 87.7 kJ mol -1 ! This is an error of 87.7 kJ mol -1 !

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Bigger Basis Sets Substantial improvements can be made in computing energies and wavefunctions by increasing the number of basis functions. Substantial improvements can be made in computing energies and wavefunctions by increasing the number of basis functions. The next step up from a minimal basis set is a so-called “split valence” basis set. The next step up from a minimal basis set is a so-called “split valence” basis set. In split valence basis sets we allow for more than one function per valence AO. In split valence basis sets we allow for more than one function per valence AO. We may have 2 or 3 or 4 etc. basis functions per valence AO. We may have 2 or 3 or 4 etc. basis functions per valence AO.

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Basis Set Terminology (1) 2 basis functions per valence AO is called a valence double zeta basis set. 2 basis functions per valence AO is called a valence double zeta basis set. 3 basis functions per valence AO is called a valence triple zeta basis set. 3 basis functions per valence AO is called a valence triple zeta basis set. 4 basis functions per valence AO is called a valence quadruple zeta basis set. 4 basis functions per valence AO is called a valence quadruple zeta basis set. May have 5, 6, or even higher numbers of basis function per valence AO. May have 5, 6, or even higher numbers of basis function per valence AO.

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Basis Set Terminology (2) Examples of valence double zeta basis sets are the 3-21G basis set or the 6-31G basis set. Examples of valence double zeta basis sets are the 3-21G basis set or the 6-31G basis set. An example of a valence triple zeta basis set is the 6-311G basis set. An example of a valence triple zeta basis set is the 6-311G basis set. The above notation is attributed to Pople and co-workers. The above notation is attributed to Pople and co-workers.

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Basis Set Terminology (3) The Pople general form for basis set notation is M-ijk…G. The Pople general form for basis set notation is M-ijk…G. M is the degree of contraction to be used for the single basis function per each core AO. M is the degree of contraction to be used for the single basis function per each core AO. The number of digits after the hyphen denotes the number of basis functions per valence AO. The number of digits after the hyphen denotes the number of basis functions per valence AO. The value of each digit denotes the degree of contraction to be used for the given valence basis function. The value of each digit denotes the degree of contraction to be used for the given valence basis function.

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Basis Set Terminology (4) E.g. 3-21G means E.g. 3-21G means Each core AO on an atom will be represented by a single contracted gaussian basis function. The degree of contraction is 3. Each core AO on an atom will be represented by a single contracted gaussian basis function. The degree of contraction is 3. This is a valence double zeta basis set as there are 2 digits after the hyphen. This is a valence double zeta basis set as there are 2 digits after the hyphen. The first valence basis function will be represented by a contracted gaussian basis function. The degree of contraction is 2. The first valence basis function will be represented by a contracted gaussian basis function. The degree of contraction is 2. The second valence basis function will be represented by a primitive gaussian. The second valence basis function will be represented by a primitive gaussian.

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Basis Set Terminology (5) E.g. 6-311G means E.g. 6-311G means Each core AO on an atom will be represented by a single contracted gaussian basis function. The degree of contraction is 6. Each core AO on an atom will be represented by a single contracted gaussian basis function. The degree of contraction is 6. This is a valence triple zeta basis set as there are 3 digits after the hyphen. This is a valence triple zeta basis set as there are 3 digits after the hyphen. The first valence basis function will be represented by a contracted gaussian basis function. The degree of contraction is 3. The first valence basis function will be represented by a contracted gaussian basis function. The degree of contraction is 3. The second and third valence basis functions will each be represented by a primitive gaussian. The second and third valence basis functions will each be represented by a primitive gaussian.

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Calculating the Number of Basis Functions STO-3G STO-3G H and He – 1 basis function. H and He – 1 basis function. Li – Ne – 1 for the core + 4 for the valence = 5 Li – Ne – 1 for the core + 4 for the valence = 5 6-31G 6-31G H and He – 2 basis functions. H and He – 2 basis functions. Li – Ne – 1 for the core + 8 for the valence = 9 Li – Ne – 1 for the core + 8 for the valence = 9 6-311G 6-311G H and He – 3 basis functions. H and He – 3 basis functions. Li – Ne – 1 for the core + 12 for the valence = 13 Li – Ne – 1 for the core + 12 for the valence = 13

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3-21G for H (1) H has no core AO’s, so there will be two s- type basis functions that will be used to describe the 1s AO of H. H has no core AO’s, so there will be two s- type basis functions that will be used to describe the 1s AO of H. We now have MO coefficients to vary. We now have MO coefficients to vary. The 1s AO will be represented as a linear combination of the two s-type basis functions. The 1s AO will be represented as a linear combination of the two s-type basis functions. We will also get an “MO” for the 2s AO We will also get an “MO” for the 2s AO 1s = c 1,1s 1 + c 2,1s 2 1s = c 1,1s 1 + c 2,1s 2 2s = c 1,2s 1 + c 2,2s 2 2s = c 1,2s 1 + c 2,2s 2

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3-21G for H (2)

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3-21G for H (3) After minimizing the value of we obtain = -0.4961986 H = 13.503 eV. After minimizing the value of we obtain = -0.4961986 H = 13.503 eV. c 1,1s = 0.37341, c 2,1s = 0.71732 c 1,1s = 0.37341, c 2,1s = 0.71732 Error = 9.97 kJ mol -1, a much better result. Error = 9.97 kJ mol -1, a much better result.

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3-21G for H (4) We also obtain a solution for the 2s AO of H. We also obtain a solution for the 2s AO of H. c 1,2s = 1.25554, c 2,2s = -1.09602 c 1,2s = 1.25554, c 2,2s = -1.09602

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Increasing the Basis Set The table below summarizes the results for increasing the number of s basis functions from 1 (minimal) through 6. The table below summarizes the results for increasing the number of s basis functions from 1 (minimal) through 6.

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Bonding When atoms bond together to form molecules, the electrons that make up the system distribute themselves throughout space and between the nuclei to produce the lowest possible overall energy of the system. When atoms bond together to form molecules, the electrons that make up the system distribute themselves throughout space and between the nuclei to produce the lowest possible overall energy of the system. Certain parts of space have higher densities of electrons, while others contain very low densities. Certain parts of space have higher densities of electrons, while others contain very low densities. Basis sets, are functions, which constrain electron densities to certain regions of space cf. H atom. Basis sets, are functions, which constrain electron densities to certain regions of space cf. H atom. In order to obtain the correct energy of the system, we require our basis functions to correctly reflect the real electron density in our system. In order to obtain the correct energy of the system, we require our basis functions to correctly reflect the real electron density in our system. Thus our basis set should allow for as much flexibility as possible in distributing our electrons around and between nuclei. Thus our basis set should allow for as much flexibility as possible in distributing our electrons around and between nuclei. At present, the best way of doing that is by varying MO coefficients. At present, the best way of doing that is by varying MO coefficients. Because of this we often need quite a few, and a wide variety of, fixed functions. Because of this we often need quite a few, and a wide variety of, fixed functions.

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More Flexibility (1) We can increase the number of functions of the same angular type, e.g., more s functions. We can increase the number of functions of the same angular type, e.g., more s functions. E.g. STO-3G → 3-21G → 6-311G … E.g. STO-3G → 3-21G → 6-311G … Adding more functions of the same l type (recall l=0 for s AO) will only allow for electrons to be further “spread out”, or for placing more “nodes” in electron density as we move away from the nucleus. Adding more functions of the same l type (recall l=0 for s AO) will only allow for electrons to be further “spread out”, or for placing more “nodes” in electron density as we move away from the nucleus.

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More Flexibility (2) Here are the 6 s functions used in the cc-pV6Z basis (more on this basis set later) for H. Here are the 6 s functions used in the cc-pV6Z basis (more on this basis set later) for H. Electron density is permitted to be more spread out, but is spherically symmetric. Electron density is permitted to be more spread out, but is spherically symmetric. There is never any special direction is space that electrons prefer to be concentrated. There is never any special direction is space that electrons prefer to be concentrated.

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Introducing Polarization We can increase the number of functions of the same angular type, e.g., more s functions. We can increase the number of functions of the same angular type, e.g., more s functions. Adding more functions of the same l type will only allow for electrons to be further “spread out” or more nodes to exist. Adding more functions of the same l type will only allow for electrons to be further “spread out” or more nodes to exist. However, it does not allow for a different directional distribution of electron density than what we already have. However, it does not allow for a different directional distribution of electron density than what we already have. We can also include higher angular types of basis functions. We can also include higher angular types of basis functions. This does allow for different preferred directions in space for electrons to wonder around in. This does allow for different preferred directions in space for electrons to wonder around in. For H this would mean allowing p-type functions and also d-types, etc., to partake in bonding. For H this would mean allowing p-type functions and also d-types, etc., to partake in bonding. For Li – Ar this would mean including d-type and also f-type etc. For Li – Ar this would mean including d-type and also f-type etc.

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Case Study: H 2 Comparing the 6-311G basis with and without polarization functions (p functions) on each H atom in H 2, we obtain the following MO coefficients. Comparing the 6-311G basis with and without polarization functions (p functions) on each H atom in H 2, we obtain the following MO coefficients. Each H atom has directed some electron density specifically toward the other H atom. Each H atom has directed some electron density specifically toward the other H atom. Each H atom has been “polarized”. Each H atom has been “polarized”.

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Basis Set Terminology (6) Polarization functions are often added separately to atoms other than H and He (atoms other than H and He are termed “heavy” atoms). Polarization functions are often added separately to atoms other than H and He (atoms other than H and He are termed “heavy” atoms). Adding 1 set of polarization functions to heavy atoms is designated by a “*” or (d) after the basis set designation. Adding 1 set of polarization functions to heavy atoms is designated by a “*” or (d) after the basis set designation. Adding 1 set of polarization functions to H and He is designated by a second “*” or a by (d,p) after the basis set designation. Adding 1 set of polarization functions to H and He is designated by a second “*” or a by (d,p) after the basis set designation. E.g 3-21G* adds a set of d-type functions to all heavy atoms in the molecule. E.g 3-21G* adds a set of d-type functions to all heavy atoms in the molecule. E.g. 3-21G** adds a set of d-type functions to all heavy atoms in the molecule and a set of p-type functions to all H and He atoms in the molecule. E.g. 3-21G** adds a set of d-type functions to all heavy atoms in the molecule and a set of p-type functions to all H and He atoms in the molecule. 3-21G(d) is synonymous to 3-21G* and 3-21G(d,p) is synonymous to 3-21G** 3-21G(d) is synonymous to 3-21G* and 3-21G(d,p) is synonymous to 3-21G** Adding two sets of d-type functions to heavies is denoted by (2d). Adding two sets of d-type functions to heavies is denoted by (2d). Adding two sets of d-type functions and a set of f-type functions to heavies, and two sets of p-type and a set of d-type to H and He is designated by (2df,2pd), etc. Adding two sets of d-type functions and a set of f-type functions to heavies, and two sets of p-type and a set of d-type to H and He is designated by (2df,2pd), etc.

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Diffuse Functions If the problem at hand suggests that electron density might be found a long way from the nuclei, then, so- called “diffuse” functions can be added. If the problem at hand suggests that electron density might be found a long way from the nuclei, then, so- called “diffuse” functions can be added. Computing anions is an example were diffuse functions are necessary. Computing anions is an example were diffuse functions are necessary. Diffuse functions are of the same type as valence functions (s and p’s for Li – Ar, or just s for H and He). Diffuse functions are of the same type as valence functions (s and p’s for Li – Ar, or just s for H and He). Diffuse functions are characterized by small basis set exponents, i.e., small values for the . Diffuse functions are characterized by small basis set exponents, i.e., small values for the . E.g., for the 6-31G basis set with diffuse functions on H, the ’s are: (18.7311,2.82539,0.640122); (0.161278); (0.036) E.g., for the 6-31G basis set with diffuse functions on H, the ’s are: (18.7311,2.82539,0.640122); (0.161278); (0.036)

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Addition of Diffuse Functions Let’s look at H and H - with diffuse functions starting with the 6-311G basis set. Let’s look at H and H - with diffuse functions starting with the 6-311G basis set. ’s are as follows: (33.865, 5.09479, 1.15879), (0.32584), (0.102741) (0.036), (0.018), (0.009), (0.0045) ’s are as follows: (33.865, 5.09479, 1.15879), (0.32584), (0.102741) (0.036), (0.018), (0.009), (0.0045) The last three exponents are simply ½ the previous exponent. The last three exponents are simply ½ the previous exponent.

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Basis Set Terminology (7) In the Pople notation, a single set of diffuse functions are added to heavy atoms by adding a “+” after the digits representing the number of valence functions. In the Pople notation, a single set of diffuse functions are added to heavy atoms by adding a “+” after the digits representing the number of valence functions. A second “+” represents a single set of diffuse functions added to H and He atoms. A second “+” represents a single set of diffuse functions added to H and He atoms. Thus a 6-31++G basis set has a single set of diffuse functions added to heavy atoms and H and He atoms. Thus a 6-31++G basis set has a single set of diffuse functions added to heavy atoms and H and He atoms.

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Example Basis Set Designation 6-311++G(2df,2pd) for benzene. 6-311++G(2df,2pd) for benzene. For C For C A single contracted GTO of degree 6 to mimic the 1s core AO. A single contracted GTO of degree 6 to mimic the 1s core AO. Three functions per valence AO, the first will be a contracted GTO of degree 3, and the remaining two will be made up of a single gaussian each. Three functions per valence AO, the first will be a contracted GTO of degree 3, and the remaining two will be made up of a single gaussian each. A set of diffuse functions will be added, i.e., a single diffuse s and a diffuse p x, p y and p z. A set of diffuse functions will be added, i.e., a single diffuse s and a diffuse p x, p y and p z. Two sets of d polarization functions will be added. Two sets of d polarization functions will be added. A single set of f functions will be added. A single set of f functions will be added. No. basis functions = 1 for the core + 4 valence AO x 3 functions for the ‘311’ part + 4 diffuse + 5 d AO x 2 + 7 f AO = 34. No. basis functions = 1 for the core + 4 valence AO x 3 functions for the ‘311’ part + 4 diffuse + 5 d AO x 2 + 7 f AO = 34. For H For H Three functions for the 1s AO, the first being a contracted GTO of degree 3, and the remaining two are simple primitives. Three functions for the 1s AO, the first being a contracted GTO of degree 3, and the remaining two are simple primitives. A diffuse s function added to them. A diffuse s function added to them. Two sets of p polarization functions added. Two sets of p polarization functions added. A single set of d polarization functions added. A single set of d polarization functions added. No. basis functions = 1 valence AO x 3 functions for the ‘311’ part + 1 diffuse + 3 p AO x 2 + 5 d AO = 15 No. basis functions = 1 valence AO x 3 functions for the ‘311’ part + 1 diffuse + 3 p AO x 2 + 5 d AO = 15 For C 6 H 6 we will therefore require a total of 34 x 6 +15 x 6 = 294 basis functions. For C 6 H 6 we will therefore require a total of 34 x 6 +15 x 6 = 294 basis functions. This is going to be a fairly big calculation! This is going to be a fairly big calculation! Still, an energy calculation on D 6h benzene takes only 5 min on a XP1000 Dec- Alpha. Still, an energy calculation on D 6h benzene takes only 5 min on a XP1000 Dec- Alpha.

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5 d OR 6 d? Because p, d, f, etc. basis functions are expressed in terms of Cartesian coordinates like Because p, d, f, etc. basis functions are expressed in terms of Cartesian coordinates like For the d functions we have a 6 possible combinations: x 2, y 2, z 2, xy, xz, yz. For the d functions we have a 6 possible combinations: x 2, y 2, z 2, xy, xz, yz. However, hydrogenic AO’s are actually expressed in-terms of spherical polar coordinates, and not Cartesians, so one can take the appropriate linear combinations of the above Cartesians to arrive at 5 functions (2z 2 - x 2 - y 2, x 2 – y 2, xy, xz, yz) instead of 6. However, hydrogenic AO’s are actually expressed in-terms of spherical polar coordinates, and not Cartesians, so one can take the appropriate linear combinations of the above Cartesians to arrive at 5 functions (2z 2 - x 2 - y 2, x 2 – y 2, xy, xz, yz) instead of 6. The missing function actually transforms as an s function, and not a d function (x 2 + y 2 + z 2 ) The missing function actually transforms as an s function, and not a d function (x 2 + y 2 + z 2 ) When using the Pople basis sets it is sometimes necessary to specify whether you wish to use the 5 d set or 6 d set. When using the Pople basis sets it is sometimes necessary to specify whether you wish to use the 5 d set or 6 d set.

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Basis sets from other workers A superb set of basis functions originates from Dunning and co- workers. A superb set of basis functions originates from Dunning and co- workers. These authors use a very simple designation scheme. These authors use a very simple designation scheme. The basis sets are designated as either: The basis sets are designated as either: cc-pVXZ cc-pVXZ aug-cc-pVXZ. aug-cc-pVXZ. The ‘cc’ means “correlation consistent”. The ‘cc’ means “correlation consistent”. The ‘p’ means “polarization functions added”. The ‘p’ means “polarization functions added”. The ‘aug’ means “augmented”, with the functions actually added being essentially diffuse functions. The ‘aug’ means “augmented”, with the functions actually added being essentially diffuse functions. The ‘VXZ’ means “valence-X-zeta” where X could be any one of the following The ‘VXZ’ means “valence-X-zeta” where X could be any one of the following ‘D’ for “double”, ‘T’ for “triple”, Q for “quadruple”, or 5 or 6, etc. ‘D’ for “double”, ‘T’ for “triple”, Q for “quadruple”, or 5 or 6, etc. Determining the number of basis functions is done by considering the valence space and placing X functions down for each valence AO with the largest value of l. Determining the number of basis functions is done by considering the valence space and placing X functions down for each valence AO with the largest value of l. We then take one less function as we go up in the l quantum number, and take an extra function as we go down in l quantum number. We then take one less function as we go up in the l quantum number, and take an extra function as we go down in l quantum number. If the basis set is an ‘aug’ type, then we add one function across the board for each l-type function we have. If the basis set is an ‘aug’ type, then we add one function across the board for each l-type function we have.

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Examples of Dunning’s cc Basis Sets cc-pVDZ for Li – Ne cc-pVDZ for Li – Ne We will have [3s2p1d], which is 3 + 2 x 3 + 1 x 5 = 14 basis functions per atom in this row. We will have [3s2p1d], which is 3 + 2 x 3 + 1 x 5 = 14 basis functions per atom in this row. Each H and He will have [2sp] = 5 functions. Each H and He will have [2sp] = 5 functions. aug-cc-pVDZ for Li - Ne aug-cc-pVDZ for Li - Ne We will have [4s3p2d], which is 4 + 3 x 3 + 2 x 5 = 23 basis functions per atom in this row. We will have [4s3p2d], which is 4 + 3 x 3 + 2 x 5 = 23 basis functions per atom in this row. Each H and He will have [3s2p] = 9 functions. Each H and He will have [3s2p] = 9 functions. cc-pV5Z for Li – Ne cc-pV5Z for Li – Ne We will have [6s5p4d3f2gh], which is 6 + 5 x 3 + 4 x 5 + 3 x 7 + 2 x 9 + 1 x 11 = 91 basis functions per atom in this row! We will have [6s5p4d3f2gh], which is 6 + 5 x 3 + 4 x 5 + 3 x 7 + 2 x 9 + 1 x 11 = 91 basis functions per atom in this row! Each H and He will have [5s4p3d2fg] = 55 functions. Each H and He will have [5s4p3d2fg] = 55 functions.

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Wondering what h AO’s look like? Check out this site: Check out this site: http://www.orbitals.com/orb/orbtable.htm http://www.orbitals.com/orb/orbtable.htm http://www.orbitals.com/orb/orbtable.htm

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Effective Core Potentials Normally applied to third and higher row elements. Normally applied to third and higher row elements. A potential replaces the core electrons in a calculation with an effective potential. A potential replaces the core electrons in a calculation with an effective potential. Eliminates the need for core basis functions, which usually require a large number of primitives to describe them. Eliminates the need for core basis functions, which usually require a large number of primitives to describe them. May be used to represent relativistic effects, which are largely confined to the core. May be used to represent relativistic effects, which are largely confined to the core. Some examples are: CEP-4G, CEP-31G, CEP- 121G, LANL2MB (STO-3G 1 st row), Some examples are: CEP-4G, CEP-31G, CEP- 121G, LANL2MB (STO-3G 1 st row), LANL2DZ (D95V 1 st row), SHC (D95V 1 st row) LANL2DZ (D95V 1 st row), SHC (D95V 1 st row)

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Basis Set Quality ECP minimal basis sets are clearly the worst quality, followed closely by minimal basis sets. ECP minimal basis sets are clearly the worst quality, followed closely by minimal basis sets. DZ basis sets are a marked improvement, but still generally of low quality. DZ basis sets are a marked improvement, but still generally of low quality. 6-311G 6-311G 6-311G(2df,2pd) ~ cc-pVTZ 6-311G(2df,2pd) ~ cc-pVTZ 6-311++G(2df,2pd) 6-311++G(2df,2pd) aug-cc-pVTZ aug-cc-pVTZ Bigger Dunning’s basis sets now win hands- down. Bigger Dunning’s basis sets now win hands- down. A simple comparison can be made by comparing the number of s, p, d, f, etc. functions between basis sets. A simple comparison can be made by comparing the number of s, p, d, f, etc. functions between basis sets.

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One last word… Unbalanced Basis Sets 3-21++G(2df,2pd) 3-21++G(2df,2pd) Only 2 functions per valence AO, Only 2 functions per valence AO, but 3 polarization functions and a diffuse?but 3 polarization functions and a diffuse? 6-311+G(2df) 6-311+G(2df) 3 functions per valence AO, 3 polarization and a diffuse on heavies, 3 functions per valence AO, 3 polarization and a diffuse on heavies, but no polarization nor diffuse on H?but no polarization nor diffuse on H? aug-cc-pV5Z on heavies, cc-pVDZ on H. aug-cc-pV5Z on heavies, cc-pVDZ on H. aug-cc-pV5Z (sp only for H-Ne). aug-cc-pV5Z (sp only for H-Ne).

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