Generalized Tamm–Dancoff approximation (GTDA) and random-phase approximation (GRPA) calculations on LiH, Be, and Li2

Abstract

The theory for polarization propagator calculations based on a correlated reference state in the form of a spin singlet restricted antisymmetrized geminal power (AGP) is discussed. Particular attention is paid to the calculation of excitation energies and transition moments for the molecules over all internuclear separations, i.e., not just single point vertical energies, which is done in standard RPA or in higher-order treatments using perturbation methods. Simple applications to the Be atom, and the LiH and Li2 molecules are used to illustrate how this simple correlated state, which is completely determined by the choice of orbital basis (i.e., no choice of configurations is necessary), can cure the notorious consistency problems and the instabilities against the breaking of symmetry, which plague standard RPA based on a Hartree–Fock (HF) single determinantal reference state.