1. Electron Correlation Hartree-Fock results do not agree with experiment
Heirarchy of methods to treat electron-electron interactions electron correlation ie what approximation do we use for H?
repulsion -
attraction -
attraction +
Hartree-Fock theory – just consider 1 electron + “average” repulsion
Need an initial guess of the average repulsion (ie the electron density)
Iterate until self-consistent

2. What Tools Can We Use? Density Functional Theory  quantum method
 in principle “exact”
 faster than traditional ab initio
 variable accuracy
 no systematic improvement
Walter Kohn, Nobel Prize 1998

3. Density Functional Theory
The energy and electronic properties of the ground state are uniquely determined by the electron density:
E = E[]
In principal this expression is exact!
But we don’t know what the functional is
Use model systems and fitting to derive expressions giving different “functionals”
The electron density is something we can “see”
The electron density is a 3-dimensional property whereas wavefunction-based methods are 3N dimensional
Using the Kohn-Sham orbitals DFT is mathematically equivalent to HF theory

4. Density Functional Theory
ET The kinetic energy
EV The Coulomb attraction of the electrons to the nucleus
EJ The Coulomb energy of that the electrons would have in their own field, assuming they moved independently and if each electron repelled itself
EX The Exchange energy
EC The Correlation energy
EXC corrects for the false assumptions in EJ

5. First Generation DFT Energy a functional of r alone
Analytic expressions derived from the uniform electron gas
Local Density Approximation
Local Spin Density Approximation
LDA functionals were originally developed for metals and assume the electron density is constant, not a sensible assumption in a molecule.
LDA tends to underestimate exchange energies by up to 10%, to overestimate correlation energies by up to a factor of two and to “overbind” molecules.
This approximate cancellation of errors made initial LDA results look so promising…

6. Second Generation DFT
Used LDA/uniform electron gas expressions for ET, EV and EJ
Invoked the generalised gradient approximation (GGA) for EXC to attempt to correct for non-local interactions, inhomogeneities in the electron gas, using the gradient of the density:
Meta functionals incorporate the local kinetic energy density, t (r), which is dependent on the Kohn-Sham orbitals:

7. Second Generation DFT
GGAs, and meta-GGAs are “local” functionals because the electronic energy density at a single spatial point depends only on the behavior of the electronic density and kinetic energy at and near that point.
Examples of second generation functionals include Becke’s 1986 exchange functional, the LYP correlation functional, and the PBE and the PW91 functionals
These functionals are commonly used in plane wave DFT calculations and in calculations on large systems

8. Third Generation DFT
Functionals where the electronic energy is a functional of the electron density, its gradient and its Laplacian, that is, E[r; r; 2r]
Hybrid functionals where a proportion of the exact HF exchange energy is included to introduces a degree of “non- local” behaviour
The most popular hybrid functional is the B3LYP functional:
where the coefficients were found empirically
Hybrid functionals generally perform better than GGA functionals in chemical applications

9. Fourth Generation DFT Meta-hybrid functionals
Double hybrid functionals
Extensively parametrised functionals….
These functionals attempt to correct for the “local” behaviour of DFT and give much better results for systems with weak or non-bonded interactions

10. DFT Performance LDA: GGA: Hybrid functionals:
Works well for anything where the uniform electron gas is a sensible model (eg metals) and for bulk properties
Not accurate for chemical applications
GGA:
Fast
Binding energies to about 20 kcal/mol
Hybrid functionals:
Slower (HF exchange costs)
3-12 kcal/mol errors
Fourth generation functionals:
Relatively expensive…
Claim to do a lot better

11. Density Functional Theory
Density Functional Theory is only marginally more expensive than HF theory.
Because it contains an estimate of the electron correlation energy it should always be used in preference to HF
HOWEVER, DFT calculations must be validated by comparison against some higher level of theory (sometimes they fail catastrophically…)

12. Model Chemistries
Theoretical models are defined by specifying a correlation procedure and a basis set
HF/STO-3G: A very simple theoretical model (level of theory)
MP2/6-31G(d): An intermediate level of theory
CCSD(T)/6-311+G(3d,2p): A high level of theory
B3LYP/6-31G(d): A cost effective level of theory
Some properties (eg geometry) can be obtained reliably at simple levels of theory
Others (eg reaction energy, reaction barrier) require a high level of theory

13. Beyond HF: Electron Correlation Methods
For a given basis set, the difference between the exact energy and the HF energy is the correlation energy, ~ 85 kJ/mol correlation energy per electron pair
Dynamic correlation: electrons repel each other and get out of each other’s way; dynamical motions of electrons are correlated, so electron repulsion is less than in an independent electron model such as HF theory
Static Electron Correlation/Non-Dynamic Electron Correlation/Intrinsic Electron Correlation: arises when a single configurational treatment (ie a single determinant) is not adequate to describe the problem (eg the ground state of the molecule)

14. Frozen Core Approximation
assume that only the valence electrons are correlated
the core orbitals are treated at the HF level of theory
This assumption is normally good for systems involving first and second row atoms
For third row, or higher, the approximation should probably be checked
for example, if you are not careful in studying a molecule like CaF2, you may find that in the Ca2+ species none of the electrons have been correlated because the 3p orbitals are considered core orbitals (and the F atoms have effectively removed the valence 4s electrons)

15. Møller-Plesset Perturbation Theory
Although correlation energy is large on a chemical scale it is small compared to the total energy of an atom
We can treat correlation as a perturbation to the HF Hamiltonian
Expand the perturbation in l:
Møller-Plesset perturbation theories, MP2, MP3, MP4… are obtained by setting l=1 and truncating at the 2nd, 3rd, 4th… order terms in l for the wavefunction (l+1 for the energy).

16. Møller-Plesset Perturbation Theory
Not a variational method
Overcorrection possible
Not appropriate if compound is not well described by a simple Lewis structure
Does not do well in cases of spin contamination
Computational effort nN4 (MP2) n3N4 (MP4) for n electrons and N orbitals
Does not converge smoothly (oscillates)
Sometimes nonconvergent series (eg Ne)
MP2 often gives better results than MP3, MP4…

17. Configuration Interaction
Mathematically we want to allow electrons in the wavefunction to be able to move together
We can re-expand the wavefunction in terms of some orthogobal basis that encapsulates this concerted movement
We can use all possible HF-SCF determinants as this basis
A determinant describes an electron configuration, “excited” determinants excite one or more electrons into unoccupied orbitals
They are all orthogonal to each other
This single, double, triple etc excitation correlates the electrons

18. Configuration Interaction
The wavefunction is expanded as a linear combination of all possible HF-SCF determinants
The CI coefficients bs are determined variationally
The size of the FCI calculation depends on the number of electrons, n, and the number of orbitals, N.
For N basis functions there are 2N spinorbitals and the total number of determinants is (2N!)/[n!(2N-n)!] ~ eN

19. Configuration Interaction
O occupied
V virtual orbitals
CIS – include all possible single electron excitations: simplest qualitative method for electronic excited states, but not for correlation of the ground state
CISD – include all single and double excitations (yields ~O2V2 determinants) most useful for correlating the ground state
CISDT – singles, doubles and triples (~O3V3 determinants)
Full CI (FCI) – (~((O+V)!/O!V!) determinants) exact for a given basis set

20. Coupled Cluster Theory
The CI expansion converges slowly
Some excitations are more important than others…
Define a cluster operator: T= 1 + T1 + T2 + T3 +…
Write Y as
Where
This is particularly clever because

21. Coupled Cluster Theory
If we truncate T
T= 1 + T1 + T2
Then eT will contain products of T1 and T2 that are equivalent to higher order excitations
T12 represents all double excitations arising from “disconnected” single excitations,
T22 represents all quadruple excitations arising from disconnected double excitations
These disconnected excitations turn out to be important (than the generic n-electron excitations) so the coupled cluster wavefunction converges much more rapidly than the CI expansion

22. CCSD(T) Truncate T T= 1 + T1 + T2 Include T3 as a perturbation
Simpler and faster and almost as accurate as CCSDT
The CCSD(T) method is the highest level theory available for routine use. With a large basis set CCSD(T) is considered the “Gold Standard” for dynamic electron correlation:
CCSD(T)/aug-ccpVTZ

23. Static Correlation
If the wavefunction is not well described as a single determinant
Species with significant diradical character
Transition States (frequently)
Bond breaking processes
Often for excited electronic states
Unsaturated transition metal complexes
molecules containing atoms with low-lying excited states (Li, Be, transition metals, etc)
along reaction paths in many chemical and photochemical reactions
Generally any species with near degeneracies
The T1 diagnostic in CC methods is an indicator of the validity of a single reference approach. T1 > 0.01 casts suspicion on the applicability of single reference methods.

24. Multi-Configuration Methods
Similar to the CI expansion
Optimise the one-electron orbitals rather than leave them at their HF values
Eg MR-CI(SD), CASSCF, CASPT2 …
Starting to get into some serious computational expense…

25. Cyclobutadiene 4 p electrons and 4 p molecular orbitals
Each diagram represents a determinant (a configuration state function)
The overall wavefunction is a combination of the possible determinants
The coefficients of the orbitals change with their occupancy (consider square vs rectangular cyclobutadiene)

26. Cyclobutadiene
Active space: 4 electrons in 4 orbitals Core inactive space: remaining 16 electrons in 8 orbitals Virtual/Unoccupied orbitals

27. Assessment of Correlated Methods
Av. Error (kcal/mol) vs FCI
Approx. Time Factor HF
5-30
ON2-3
DFT
2-10
ON3
MP2
17.4
ON4
MP3
14.4
MP4
3.7
ON5
MP5
3.2
CISD
13.8
ON6
CCSD
4.4
CCSD(T)
0.7
O2N7
CCSDT
0.5
O2N>7
CCSDTQ
0.0
O2N>>7

28. Model Chemistries
Choosing a method (theoretical model) in ab initio calculations involves striking a compromise between accuracy and computational expense – the more reliable the calculations generally the more computationally demanding
The method chosen depends on
The size of the molecule being examined
The property being calculated
The accuracy that is required
The computing resources that are available

29. Exact Soln of Schrödinger Equation
Pople Diagram
John Pople
Nobel Prize 1998
A specific level of theory (theoretical model) corresponds to a combination of correlation procedure and basis set
Improvement of Correlation Treatment
Improvement of Basis Set
HF MP2 MP4 CCSD(T)
Full Configuration Interaction
Exact Soln of Schrödinger Equation
Completely Flexible Basis Set
STO-3G
3-21G
6-31G(d)
6-311+G(2df,p)
The better you do the longer it takes

30. Composite Methods
Extrapolate to the bottom right corner of the Pople diagram
Aim is better than 1 cal/mol accuracy
Gaussian “n” methods
G1, G2, G2MP2, G3…
Complete Basis Set Limit (CBS) methods
Weizmann Wn methods
HEAT method…

31. Summary
Computational chemistry can be used to predict molecular properties, such as:
Equilibrium geometries
Transition structures
Reaction potential energy surfaces
Many tools are available. In general, the more accurate the method the more costly it is to use
Before using a particular approach and methodology, you need to make sure it is accurate enough for your particular problem

32. Some Observations Chemists like simple systems
Chemists are interested in electrons so they tend to use the most accurate methods they can
Big problems need to be distilled into small enough bits to provide sensible results
Everything kicks up more questions, nothing is ever as simple as it seems