The potential of Potential Functional Theory

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1 The potential of Potential Functional Theory IPAM DFT School 2016 Attila Cangi August, Max Planck Institute of Microstructure Physics, Germany P. Elliott, AC, S. Pittalis, E.K.U. Gross, K. Burke, PRA 92, (2015).

2 Going beyond DFT Electronic structure problem What atoms, molecules and solids can exist? What properties (ground-state energy, electron density geometry, bond distances, angles, nuclear vibrations, ionization energies, bond dissociation, etc.) do they have? PFT is an alternative approach Enables ab-initio calculations of very large molecular systems. Avoids most-costly step by approximating the KS kinetic and exact exchange energy as a functional of the KS potential. Combines elements of today s lectures on Hohenberg-Kohn theorem, KS equations, Local density approximation, Density matrices and holes.

3 Hamiltonian and ground-state energy Atomic units throughout (e 2 = = m e = 1); suppress spin indices. Ground-state energy of N electrons: where E 0 = min Ψ Ĥ Ψ = min Ψ ˆT + ˆV ee + ˆV Ψ, Ψ Ψ ˆT = 1 2 j, ˆVee = r j j j k j r k, ˆV = v(r j ), j and Ψ are N-particle wave functions that are antisymmetric, normalized, and have finite kinetic energy.

4 Universal functional in PFT Use Hohenberg-Kohn mapping for interacting and noninteracting particles: v(r) n(r) v S (r). Write universal part of Hohenberg-Kohn functional as functional of the KS potential: F [ṽ S ] = T S [ṽ S ] + U[ṽ S ] + E XC [ṽ S ]. Obtain true gs energy from the variational principle ( ) E 0 = min F [ṽ S ] + dr n S [ṽ S ](r) v(r). ṽ S

5 Self-consistent equations At the minimum the gs energy satisfies δe 0 [ṽ S ] δṽ S (r) = 0. vs Given n S [v S ](r) and E XC [v S ], v S[v S ](r) = v(r) + dr χ 1 S [v S ](r, r) δe HXC[ṽ S ] δṽ S (r ), vs v S[v S ](r) = dr χ 1 S [v S ] δt S[ṽ S ] δṽ S determine the minimizing v S (r), and hence, the true gs energy, where vs χ S [v S ](r, r) = δn S [ṽ S ](r )/δṽ S (r) vs denotes the one-body density-density response function.

6 KS kinetic energy Introduce a coupling-constant λ [0, 1] in the KS potential: v λ S (r) = λ v S (r) + (1 λ) v R (r), v S (r) = v S (r) v R (r). Obtain the KS kinetic energy just from the density as a functional of the KS potential T S [v S ] = E R S + dr { n S (r) v S (r) n S [v S ](r) v S (r)}, where n S [v S ](r) = 1 0 dλ n S[v λ S ](r).

7 Exchange energy Obtain the exchange energy through the 1RDM: E X [v S ] = dr dr γs[v S ](r, r )γ S [v S ](r, r ) v ee (r, r ). The corresponding exchange hole is defined as: n X (r, r ) = γ S[v S ](r, r )γ S [v S ](r, r ) n(r) obeying the sum rule dr n X (r, r ) = 1 r.,

8 Up to now everything was exact.

9 Semiclassical approxmiation to the 1RDM γ sc S (x, x ) = λ sin[θf λ(x, x )]cosec[αf λ(x, x )/2] 2T λ=± F kf (x)k F (x ). Classical momentum: k(x) = 2 (ɛ v S (x)). Classical action: θ(x) = x 0 dx k(x ), θ F (L) = (N + 1/2)π. Classical time: t(x) = x 0 1 k(x ), T = t(l), α(x) = πt(x)/t. Abbreviations: θ ± (x, x ) = θ(x) ± θ(x ), α ± (x, x ) = α(x) ± α(x ).

10 Semiclassical exchange hole

11 Non-variational calculations Error (a.u.) LDAX scx* sckx* N Figure 1: Energy error made by LDA exchange (LDAX), non-variational semiclassical exchange (scx*), and semiclassical kinetic and exchange (sckx*) for N spin-unpolarized, interacting fermions in a 1d well.

12 Variational calculations Table 1: Total EXX energy and respective errors of variational calculations within LDAX, scx, and sckx for N spin-unpolarized fermions interacting via exp( 4u) in an external potential v(x) = 5 sin 2 (πx) within a box of unit length. N E EXX E EXX X error 10 3 LDAX scx sckx

13 Conclusions Our exchange approximation (scx) is as accurate as EXX and comes at almost no cost. Our orbital-free, pure PFT calculation (sckx) has comparable accuracy and comes at even smaller computational cost. Analogous formulas highly desired for potentials in 3D.

14 Appendix: LDA exchange in 1D The (spin-polarized) LDAX energy per electron in 1D: ɛ LDA X (n(x)) = arctan β + ln(1 + β2 ), β = 2π n/α. π 2πβ

15 Appendix: Non-variational calculations (energetics) Non-variational, semiclassical exchange approximation evaluated on KS potential from LDAX. Table 2: Total EXX energy and respective errors of perturbative post-ldax(*) calculations within LDAX, scx, and sckx for N spin-unpolarized fermions interacting via exp( 4u) in an external potential v(x) = 5 sin 2 (πx) within a box of unit length. N E EXX E EXX X error 10 3 LDAX scx* sckx*

16 Appendix: Variational calculations (KS potentials) n(x) (a.u.) KS potential (a.u.) EXX LDAX scx sckx x (a.u.) Figure 2: Upper plot: Converged KS potentials of EXX, LDAX, scx, and sckx runs. Lower plot: Error in the respective, converged densities with respect to EXX.

17 Appendix: Variational calculations (energy density) E X energy density (a.u.) error (a.u.) EXX LDAX scx sckx x (a.u.) Figure 3: Exchange energy density (upper) and its error (lower).

18 Appendix: Variational calculations (energy components) Table 3: Energy components of variational calculations within LDAX, semiclassical exchange (scx), and a semiclassical approximation of all energy components (sckx) for 4 electrons in the same problem as in Tab. 2. EXX error 10 3 LDAX scx sckx E T S V ext U E X

Introduction to Density Functional Theory

Introduction to Density Functional Theory S. Sharma Institut für Physik Karl-Franzens-Universität Graz, Austria 19th October 2005 Synopsis Motivation 1 Motivation : where can one use DFT 2 : 1 Elementary