Density Functional Theory (DFT) DFT is an alternative approach to the theory of electronic structure; electron density plays a central role in DFT. Why.

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Density Functional Theory (DFT) DFT is an alternative approach to the theory of electronic structure; electron density plays a central role in DFT. Why a new theory? HF method scales asK 4 (K - # of basis functions) CI methods scale asK 6 -K 10 MPn methods scale as>K 5 CC methods scale as >K 6 Correlated methods are not feasible for medium and large sized molecules! The electron density - it is the central quantity in DFT - is defined as: Alternative: DFT

The electron density

Properties of the electron density Function: y=f(x)ρ= ρ(x,y,z) Functional: y=F[f(x)]E=F[ρ(x,y,z)]

First HK Theorem: Hohenberg–Kohn Theorems The external potential V ext (r) is (to within a constant) a unique functional of ρ(r). Since, in turn V ext (r) fixes H, the full many particle ground state is a unique functional of ρ(r). Thus, the electron density uniquely determines the Hamiltonian operator and thus all the properties of the system.

Ψ’ as a test function for H: Ψ as a testfunction for H’: Summing up the last two inequalities: Proof: by reductio ad absurdum Contradiction!

Variational Principle in DFT Second HK Theorem The functional that delivers the ground state energy of the system, delivers the lowest energy if and only if the input density is the true ground state density. - variational principle For any trial density ρ(r), which satisfies the necessary boundary conditions such as: ρ(r)  0 and and which is associated with some external potential V ext, the energy obtained from the functional of F HK represents an upper bound to the true ground state energy E 0.

Thomas-Fermi model (1927) The explicit form of T[ρ] and E non-cl [ρ] is the major challenge of DFT F HK ??? Next step Only J[ρ] is known! L.H. Thomas, Proc. Camb. Phil. Soc., 23, (1927) E. Fermi, Rend. Acad., Lincei, 6, (1927)

T[ρ]– kinetic energy of the system Kohn and Sham proposed to calculate the exact kinetic energy of a non-interacting system with the same density as for the real interacting system. T KS – kinetic energy of a fictitious non-interacting system of the same density ρ(r) Ψ i - are the orbitals for the non-interacting system (KS orbitals) T=T KS +(T-T KS ) E xc [ρ] includes everything which is unknown: -exchange energy -correlation energy -correction of kinetic energy (T-T KS )

Minimize E[ρ] with the conditions: Kohn-Sham Equations: with:

Kohn-Sham Formalism Hartree-Fock equations Kohn-Sham equations

E xc [ρ] = ?? Local Density Approximation (LDA) ε xc only depends on the density at r Generalized Gradient Approximation (GGA) ε xc depends on the density and its gradient at r For the correlation part: Monte-Carlo simulations – Ceperly and Alder Good for solids (1)Adjust ε xc such that it satises all (or most) known properties of the exchange- correlation hole and energy. PW91, PBE… (2) Fit ε xc to a large data-set own exactly known binding energies of atoms and molecules. BLYP, OLYP, HCTH…

Hybrid Functionals Meta-GGAs No major improvements! E X KS -the exact exchange calculated with the exact KS wave function α - fitting parameter

Exchange and Correlation Functionals In practice: BLYP, B3LYP, BPW91, …

MO5-2X - bond dissociation energies, stacking and hydrogen-bonding interactions in nucleobase pairs Different functionals for different properties Atomization energies: Ionization energy: - B3LYP – the best! Electron afinities: Vibrational frequencies: - (BLYP), B3LYP, …

Kohn-Sham orbitals