Unitary group based state specific open-shell-singlet coupled-cluster method: Application to ozone and comparison with Hilbert and Fock space theories

Abstract

Recently introduced state-specific coupled-cluster method, which exploits the formalism of the unitary group approach to the many-electron correlation problem and enables a properly spin-adapted treatment of open-shell states, is applied to several low lying singlet and triplet electronic states of ozone at the double-zeta plus polarization level of approximation. The method employs a nonstandard cluster Ansatz, based on a single spin-free reference built from either the ground state restricted Hartree–Fock orbitals or from the restricted open-shell Hartree–Fock orbitals, specific for each excited state. The results are compared with available experimental data and with other ab initio calculations, particularly with those employing spin-orbital based, spin nonadapted multireference coupled-cluster approaches of both state universal and valence universal types, as well as equation-of-motion coupled-cluster method. The general agreement is satisfactory except for the B2 states, where the difference between the multireference spin nonadapted and our state specific, but spin adapted, approaches amounts to as much as 0.64 eV. It is shown that this difference arises due to the spin contamination that is present in the spin-orbital based multireference approaches.