Second-Order Multireference Algebraic Diagrammatic Construction Theory for Photoelectron Spectra of Strongly Correlated Systems

Abstract

We present a second-order formulation of multi-reference algebraic diagrammatic construction theory [Sokolov, A. Yu. J. Chem. Phys. 2018, 149, 204113] for simulating photoelectron spectra of strongly correlated systems (MR-ADC(2)). The MR-ADC(2) method uses second-order multi-reference perturbation theory (MRPT2) to efficiently obtain ionization energies and spectral densities for many photoelectron transitions in a single computation. In contrast to conventional MRPT2 methods, MR-ADC(2) provides information about ionization of electrons in all orbitals (i.e., core and active) and allows to compute transition properties in straightforward and efficient way. Although equations of MR-ADC(2) depend on four-particle reduced density matrices, we demonstrate that computation of these large matrices can be completely avoided without introducing any approximations. The resulting MR-ADC(2) implementation has a lower computational scaling compared to conventional MRPT2 methods. We present results of MR-ADC(2) for photoelectron spectra of small molecules, carbon dimer, and equally-spaced hydrogen chains (H 10 and H 30 ) and outline directions for future developments.

Simulating X-ray photoelectron spectra with strong electron correlation using multireference algebraic diagrammatic construction theory

“…For more details, we refer the readers to our previous publications. [67][68][69][70] Ionization of a many-electron system irradiated with light of frequency o is described by the backward component of the one-particle Green's function, 92,93 G pq (o):…”

“…[73][74][75][76][77][78] In this work, we use two MR-ADC methods: the strict secondorder approximation (MR-ADC(2)) and its extended version (MR-ADC(2)-X). 68,69 Fig. 1 shows the perturbative structure of the effective Hamiltonian matrix M for both methods where each matrix element corresponds to a pair of excitations.…”

“…In this work, we present a new approach for the XPS simulations of strongly correlated systems that combines multireference algebraic diagrammatic construction theory (MR-ADC) [67][68][69][70] with a core-valence separation technique (CVS). 71,72 The MR-ADC approach is naturally suited for the simulations of core-level excitations combining several attractive features: (i) low computational cost (similar to that of multireference perturbation theories), [73][74][75][76][77][78] (ii) Hermitian equations, and (iii) the ability to calculate excitations from all molecular orbitals, including inner-shell and core.…”

Third-order algebraic diagrammatic construction theory for electron attachment and ionization energies: Conventional and Green’s function implementation

“…Another important direction is to improve accuracy of EA-/IP-ADC for systems with unpaired electrons and moderate or strong electron correlation, for example, by developing multi-reference formulations of these methods. 45,50 Work along these directions is ongoing in our group.…”

“…37,41 For computations of IP of strongly correlated systems, a multi-reference formulation of IP-ADC (2) has been recently implemented in our group. 45 Two implementations 1 of the non-Dyson EA-ADC(n) (n = 2, 3) approximations have been reported, 46,47 although performance of these methods has not been documented thoroughly in the literature.…”