Multireference many‐body perturbation theory calculations on negative ions with small electron detachment energies

Abstract

ABSTRACT:We present a simple method of ab initio calculations on the systems composed of a closed-shell-like molecule (atom) and a weakly bound electron via constructing a state-specific effective Hamiltonian in a model space generated by placing the extra electron on virtual self-consistent field orbitals of the neutral species in all the possible ways. Correlation contributions are incorporated into the effective Hamiltonian by the second-order multipartitioning many-body perturbation theory. The method prevents the appearance of intruder states without destroying the exact size consistency of results and ensures a proper description of the dynamic correlation effects on the spatial localization of the extra electron and continuous behavior of computed characteristics under variations of molecular geometry. A detailed study of perturbative solutions for a model problem with parameter-dependent Hamiltonian and test calculations of Ca, Sr, and Ba atomic anions are reported. The method is also applied to the description of the hypothetical symmetrical isomer of water dimer anion with the extra electron trapped between the water molecules.

Multipartitioning many-body perturbation theory calculations on temporary anions: applications to N 2and CO

“…We briefly recall the general scheme of MPPT calculations on a system composed of a closedshell-like neutral (N-electron) subsystem and a weakly bound electron proposed in [8]. This approach is based on the construction of the many-electron state-specific effective Hamiltonian H in a model space L P spanned by the determinants | p = a † pα | 0 , where | 0 is the closed-shell SCF determinant and p runs over all the indices of canonical empty orbitals of the neutral species.…”

“…In a recent work [8] we have formulated a simple method for computing groundstate properties of an anionic system composed of a closed-shell neutral molecule and a weakly bound electron. This method is based on the use of the second-order many-body multipartitioning perturbation theory (MPPT, [9]) for correlation treatment and ensures exact size extensivity of energy estimates as well as a proper description of correlation interferences which manifest themselves as the effect of dynamic correlations on the spatial charge distribution associated with the extra electron.…”

Multipartitioning Møller–Plesset perturbation theory: Size‐extensivity at third order and symmetry conservation

“…Møller–Plesset partitioning for MR PT is straightforward to apply provided that one defines a one‐particle energy set. Several experiments on different definitions for orbital energies had the common experience that it is hard to make a suitable choice 9–11, 13, 14. Close to zero energy denominators notoriously appear and cause singularities on potential energy surfaces.…”

Ab initio study of temporary anions of benzene and fluorobenzenes using the multipartitioning many-body perturbation theory

“…As shown in ref. 17, such definition of energy denominators guarantees size extensivity of the method and justifies the use of conventional second-order Møller-Plesset perturbation theory (MP2) for obtaining consistent estimates of neutral state energies. The described formulation may resemble the equations-of-motion based approach developed in ref.…”

“…Recently 17,19 we have proposed a new version of the second order many-body multipartitioning perturbation theory (MPPT) 20 suited for the study of temporary anions decaying into a closed-shell-like molecule and an electron in the frame of the stabilization technique. In what follows this version of the theory is referred to as MPPT-R.…”