Faddeev Random Phase Approximation (FRPA) Application to Molecules

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1 Faddeev Random Phase Approximation (FRPA) Application to Molecules Matthias Degroote Center for Molecular Modeling (CMM) Ghent University INT 2011 Spring Program Fermions from Cold Atoms to Neutron Stars: Benchmarking the Many-Body Problem

2 Outline 1 Introduction 2 Model ADC(3) From TDA to RPA FRPA Faddeev equations 3 Results Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 2 / 23

3 What is FRPA? It is a Green s function method that uses Faddeev equations to couple Random Phase Approximation (RPA) interactions in a consistent way and to sum them to an infinite order Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 3 / 23

4 Situating the problem Hamiltonian Within the Born-Oppenheimer approximation Ĥ = ˆT + Ûext + ˆV ee only 2-body interactions interactions are known ˆT kinetic energy Û ext central potential due to nuclei Coulomb-repulsion between electrons ˆV ee Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 4 / 23

5 Situating the solution Green s functions In the time domain G α,β (t t ) = i Ψ N 0 T ( ) a α (t)a β (t ) Ψ N 0 propagation of single partice states from time t to t Lehmann-representation G α,β (E) = Ψ N 0 aα ΨN+1 n Ψ N+1 n a β Ψ N 0 n E (En N+1 E0 N )+iη Ψ N 0 a β Ψ N 1 N 1 m Ψ m a α Ψ N 0 m E (E0 N N 1 Em ) iη transition amplitudes excitation energies Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 5 / 23

6 Perturbation expansion Dyson equation G(E) = G 0 (E) + G 0 (E)Σ (E)G(E) Irreducible Self-Energy and higher-order vertex functions Σ (E) = Σ HF + M(E) 4-point function Σ = + Γ 4pt Examples: GW, Bold Diagrammatical Monte Carlo, T-matrix 6-point function Σ = Γ 6pt Examples: ADC(3), FRPA, all the previous examples Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 6 / 23

7 Algebraic diagrammatic construction Algebraic diagrammatic construction Schirmer et al. Phys. Rev. A 28 (1983) M(E) = U 1 E K C+iη U only depends on 1 energy expand U and C in orders U = U (1) + U (2) +... C = C (1) + C (2) +... exact up to certain order formulated as an expansion in Coulomb matrix elements coupling to 1p2h, 2p3h,... Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 7 / 23

8 Algebraic diagrammatic construction Matrix structure Indirect coupling of 2p1h and 2h1p matrix size grows every 2 orders Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 8 / 23

9 ADC(3) Third order ADC coupling between single particle states and 2p1h/2h1p consisent up to third order compromise between speed and precision good ground state energies and ionization energies Interpretation equation of motion (EOM) Qp n 1 p 2 h = a p 1 a p 2 a h diagrammatic infinite resummation interactions between two of the three lines only 3 states at any energy Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 9 / 23

10 TDA and RPA ADC(3) is equivalent to exchange of Tamm Dancoff (TDA) phonons not clear how! TDA approximation for ph and 2p propagator no mixture between forward and backward propagation RPA dynamically screens interaction used in condensed matter physics links forward and backward propagating amplitudes ( ) ( ) ( ) A B X X B = sgn (ɛ) ɛ C Y Y Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 10 / 23

11 Advantages of RPA over TDA Random Phase Approximation improves over TDA by including mixture between positive and negative energy excitations example: plasmon pole in polarization propagator Continuum Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 11 / 23

12 Diagrammatic content of FRPA Dyson equation Σ = R FRPA 2p1h propagator R = no longer only 3 lines at every energy Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 12 / 23

13 Different approach Difficulties no equivalent nth order expansion in Coulomb elements not just replacing interactions by screened RPA interactions Double counting Γ pp = Γ pp no simple Bethe Salpeter equation possible Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 13 / 23

14 Introduction of Faddeev equations Splitting up the propagator from 3-body problem but without three-body forces R(E) = R free (E) + i=1..3 R(i) (E) Interpretation of the parts R (i) = Γ (i) ( ) + + R (j) Line i propagates freely, Γ (i) is appropriate RPA interaction between line j and k R (k) Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 14 / 23

15 Faddeev equations Matrix equations Lehmann-representation for R (i) amplitudes and poles are found from eigenvalue-equation reduces to ADC(3) or FTDA when H (i) = 0 Spurious solutions 3 times matrix dimension of 2p1h-space 2/3 are spurious i=1..3 R(i) (E) = 0 Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 15 / 23

16 FRPA recipe FRPA properties infinite summation of diagrams through self-consistent equation conserving in Baym-Kadanoff manner ph- and pp-channels treated equivalently non-hermitian eigenvalue equation for 2p1h/2h1p amplitudes and energies complex eigenvalues are possible iterative treatment of static self-energy F = same U as in ADC(3) p/h 2p1h 2h1p p/h ɛ Ũ Ũ 2p1h Ũ ɛ Fd 0 2h1p Ũ 0 ɛ Fd Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 16 / 23

17 Calculations Calculated properties for linear molecules E ground-state energies vertical ionization energies bond lengths r Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 17 / 23

18 Ground state properties Molecule FTDA FRPA CCSD(T) FCI Expt. H 2 E r H H I HF E r H F I HCl E r H Cl I BF E r B F I BeH 2 E r Be H I H 2 O E r H O Λ O H O I calculated in cc-pvdz Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 18 / 23

19 Ground state properties Molecule FTDA FRPA CCSD(T) FCI Expt. N 2 E r N N I CO E r C O I CO 2 E r C O I C 2 H 2 E r C C r C H I calculated in cc-pvdz Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 19 / 23

20 Vertical ionization energies Molecule HF Level HF FTDA FRPA Expt. 1π σ CO 5σ π σ N 2 3σ g π u σ u H 2 O 1b a b (ev) 1.26 (1.14) 0.27 (0.28) 0.31 (0.26) max (ev) 2.47 (2.27) 0.68 (0.68) 0.88 (0.62) calculated in aug-cc-pvdz at experimental geometry Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 20 / 23

21 Problems in the dissociation limit FRPA FRPAc FTDA FTDAc g.s. energy H2 minimal basis ph Excitation Energy TDA ph excitation energy RPA ph excitation energy E (Hartree) E (Hartree) R (Angstrom) R (Angstrom) RPA instability H 2 in minimal basis set essentially the same as 2-site Fermi-Hubbard at half-filling ground state is combination of Slater-determinants ph excitation lower in energy Fermi-Hubbard TDA linear behavior becomes negative RPA square root behavior becomes complex Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 21 / 23

22 Conclusion Dissociation limit not feasible only good in situations where Hartree-Fock is reasonable may be solved by including fragmentation Dewulf et al. Phys. Rev. C 65 (2002) use of other phonons In general no over-correlation works for nuclei, atoms, molecules should work for extended systems (electron gas, H-chains, schematic models,...) can be improved by better self-consistency speed can be improved by partial diagonalization with Lanczos-algorithm Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 22 / 23

23 References adc(3) J. Schirmer, L.S. Cederbaum, O. Walter, Phys. Rev. A 28, 1237 (1983) theory C. Barbieri, W.H. Dickhoff, Phys. Rev. C 63, (2001) nuclei C. Barbieri, W.H. Dickhoff, Phys. Rev. C 65, (2002) atoms C. Barbieri, D. Van Neck, W.H. Dickhoff, Phys. Rev. A 76, (2007) molecules M. Degroote, D. Van Neck, C. Barbieri, Phys. Rev. A 83, (2011) Matthias Degroote (Ghent University) FRPA: Application to Molecules INT 2011 Spring Program 23 / 23