Excitation energies from relativistic coupled-cluster theory of general excitation rank: Initial implementation and application to the silicon atom and to the molecules $X$H ($X$ = As, Sb, Bi)

Excitation energies from relativistic coupled-cluster theory of general excitation rank: Initial implementation and application to the silicon atom and to the molecules $X$ H ( $X$ = As, Sb, Bi)

Mickaël Hubert, Lasse K. Sørensen, Jeppe Olsen, and Timo Fleig

Phys. Rev. A 86, 012503 – Published 13 July 2012

Abstract

We present an implementation of four-component relativistic coupled-cluster theory for the treatment of electronically excited states of molecules containing heavy elements, allowing for a consistent and accurate treatment of relativistic effects such as the spin-orbit interaction and electron correlations as well as their intertwining. Our approach uses general excitation ranks in the cluster operator and, moreover, allows for the definition of active-space selected excitations of variable excitation rank. Initial applications concern the silicon atom and the heavier pnictogen monohydride molecules, where we focus on the first vertical excitation energy to the $Ω = 1$ electronic state. We discuss the problem of adequately choosing a reference state (Fermi vacuum) and addressing electron correlation in the presence of effects of special relativity of increasing importance. For the heaviest homolog, BiH, where dynamic electron correlation is of major importance, we obtain vertical excitation energies with a deviation of less than 1% from the experimental value.