Efficient methods for computing exchange-correlation potentials for orbital-dependent functionals Viktor N. Staroverov Department of Chemistry, The University of Western Ontario, London, Ontario, Canada IWCSE 2013, Taiwan National University, Taipei, October 14β17, 2013 Orbital-dependent functionals πΈXC π = π ππ ππ« Kohn-Sham orbitals • More flexible than LDA and GGAs (can satisfy more exact constraints) • Needed for accurate description of molecular properties 2 Examples • Exact exchange πΈXexact 1 π =− 4 π ππ« π,π=1 ∗ ∗ ′ ′ π π« π π« π π« π π« π π π π ππ« ′ π« − π«′ same expression as in the HartreeβFock theory • Hybrids (B3LYP, PBE0, etc.) • Meta-GGAs (TPSS, M06, etc.) 3 The challenge KohnβSham potentials corresponding to orbitaldependent functionals π£XC πΏπΈXC [{ππ }] π« = =? πΏπ(π«) cannot be evaluated in closed form 4 Optimized effective potential (OEP) method Find π£XC (π«) as the solution to the minimization problem πΏπΈtotal =0 πΏπ£XC (π«) OEP = functional derivative of the functional 5 Computing the OEP Expand the KohnβSham orbitals: π ππ π« = πππ ππ (π«) π=1 orbital basis functions Expand the OEP: π π£XC π« = ππ ππ (π«) π=1 auxiliary basis functions Minimize the total energy with respect to {πππ } and {ππ } 6 Attempts to obtain OEP-X in finite basis sets size 7 I. First approximation to the OEP: An orbital-averaged potential (OAP) Define operator π’XC such that πΏπΈXC [{ππ }] π’XC ππ π« = πΏππ∗ (π«) The OAP is a weighted average: π£XC π« = π ∗ π π=1 π π« π’XC ππ π ∗ π π=1 π (π«)ππ π« π« 8 Example: Slater potential Fock exchange operator: πΏπΈXexact πΎππ π« ≡ πΏππ∗ (π«) Slater potential: 1 π£S π« = π π« π ππ∗ (π«)πΎππ (π«) π=1 9 Calculation of orbital-averaged potentials • by definition (hard, functional specific) • by inverting the KohnβSham equations (easy, general) 10 KohnβSham inversion KohnβSham equations: 1 2 − ∇ + π£ + π£H + π£XC ππ = ππ ππ 2 multiply by ππ∗ , sum over i, divide by π ππΏ 1 + π£ + π£H + π£XC = π π π ππ ππ 2 π=1 11 LDA-X potential via Kohn-Sham inversion 12 PBE-XC potential via KohnβSham inversion 13 Removal of oscillations A. P. Gaiduk, I. G. Ryabinkin, VNS, JCTC 9, 3959 (2013) 14 KohnβSham inversion for orbitalspecific potentials Generalized KohnβSham equations: 1 2 − ∇ + π£ + π£H + π’XC ππ = ππ ππ 2 same manipulations ππΏ 1 + π£ + π£H + π£XC = π π π ππ ππ 2 π=1 15 Example: Slater potential through KohnβSham inversion 1 2 π» π π« − π(π«) + π£S π« = 4 π π« π π=1 ππ |ππ (π«)|2 − π£ π« − π£H (π«) where 1 π= 2 π |π»ππ π=1 |2 1 2 = ππΏ + π» π 4 16 Slater potential via KohnβSham inversion 17 OAPs constructed by KohnβSham inversion 18 Correlation potentials via KohnβSham inversion 19 KohnβSham inversion for a fixed set of HartreeβFock orbitals Slater potential: π£SHF = −ππΏHF + π HF HF |2 π π=1 π |ππ πHF − π£ − π£HHF π HF |2 π |π π=1 π π πHF − π£ − π£HHF But if ππOEP ≈ ππHF , then π£XOEP ≈ π£Xmodel = −ππΏHF + 20 Dependence of KS inversion on orbital energies 21 II. Assumption that the OEP and HF orbitals are the same The assumption ππ = ππHF leads to the eigenvalue-consistent orbitalaveraged potential (ECOAP) π£XECOAP = π£SHF 1 + HF π π (ππ − ππHF ) HF 2 ππ π=1 22 ECOAP ≈ KLI ≈ LHF 23 Calculated exact-exchange (EXX) energies πΈEXX − πΈOEP , mEh m.a.v. KLI ELP=LHF=CEDA ECOAP 2.88 2.84 2.47 Sample: 12 atoms from He to Ba Basis set: UGBS A. A. Kananenka, S. V. Kohut, A. P. Gaiduk, I. G. Ryabinkin, VNS, JCP 139, 074112 (2013) 24 III. HartreeβFock exchangecorrelation (HFXC) potential An HFXC potential is the π£XC (π«) which reproduces a HF density within the KohnβSham scheme: 1 2 − ∇ + π£ π« + π£H π« + π£XC π« ππ (π«) = ππ ππ (π«) 2 That is, π£XC (π«) is such that π π π« = π ππ π« π=1 2 ππHF = π« 2 = πHF (π«) π=1 25 Inverting the Kohn–Sham equations KohnβSham equations: 1 2 − ∇ + π£ + π£H + π£XC ππ = ππ ππ 2 multiply by ππ∗ , sum over i, divide by π ππΏ 1 + π£ + π£H + π£XC = π π π ππ ππ 2 π=1 local ionization potential 26 Inverting the Hartree–Fock equations HartreeβFock equations: 1 2 − ∇ + π£ + π£H + πΎ ππHF = ππHF ππHF 2 same manipulations ππΏHF πHF + π£ + π£H + π£SHF 1 = HF π π ππHF ππHF 2 π=1 Slater potential built with HF orbitals 27 Closed-form expression for the HFXC potential HF π£XC 1 HF = π£S + π π π=1 1 2 ππ |ππ | − HF π π π=1 HF HF 2 ππ ππ π HF π + HF − π π Here π = πHF , but ππ ≠ ππHF , ππ ≠ ππHF , and π ≠ π HF We treat this expression as a model potential within the KohnβSham SCF scheme. Computational cost: same as KLI and BeckeβJohnson (BJ) 28 HFXC potentials are practically exact OEPs! Numerical OEP: Engel et al. 29 30 31 HFXC potentials can be easily computed for molecules Numerical OEP: Makmal et al. 32 Energies from exchange potentials πΈEXX − πΈOEP , mEh m.a.v. KLI LHF BJ 1.74 1.66 5.30 Basisset OEP 0.12 HFXC 0.05 Sample: 12 atoms from Li to Cd Basis set for LHF, BJ, OEP (aux=orb), HFXC: UGBS KLI and true OEP values are from Engel et al. I. G. Ryabinkin, A. A. Kananenka, VNS, PRL 139, 013001 (2013) 33 Virial energy discrepancies For exact OEPs, πΈvir − πΈEXX = 0, where πΈvir = ∫ π£X π« 3π π« + π« ⋅ ∇π(π«) ππ« πΈvir − πΈEXX , mEh KLI m.a.v. 438.0 LHF BJ 629.2 1234.1 Basis-set HFXC OEP 1.76 2.76 34 HFXC potentials in finite basis sets 35 Hierarchy of approximations to the EXX potential π£X 1 HF = π£S + HF π π ππ − ππHF ππHF π=1 HF − π π 2 + πHF OAP ECOAP HFXC 36 Summary • Orbital-averaged potentials (e.g., Slater) can be constructed by KohnβSham inversion • Hierarchy or approximations to the OEP: OAP (Slater) < ECOAP < HFXC • ECOAP Slater potential ο» KLI ο» LHF • HFXC potential ο» OEP • Same applies to all occupied-orbital functionals 37 Acknowledgments • Eberhard Engel • Leeor Kronik for OEP benchmarks 38