Chapter 6 - OEP Orbitals as a Reference for Ab Initio Many-Body Calculations

Abstract

Abstract Second-order many-body perturbation theory (MBPT) and coupled-cluster (CC) up to triple excitations (CCSDT) ab initio calculations are performed using different exchange-only references, i.e., canonical Hartree–Fock (HF), optimized effective potential (OEP), and standard Kohn–Sham (KS) density functional theory (DFT) determinants. The performance of the methods is tested for a few atomic (He, Be, Ne, Mg, Ar) and molecular ( He 2 , H 2 O , N 2 , CO ) systems, mainly in terms of total and correlation energies. In all cases, including quasi-degenerate model systems, the convergence rate of CC methods is also examined. The correlation and total energies calculated in MBPT and CC methods strongly depend on the reference orbitals. The CC ab initio calculations based on HF and exchange-only OEP orbitals lead in most cases to very close results, while use of the exchange-only LDA and PW91 significantly overestimate the MBPT and CC correlation energies. The convergence of the CC calculations is virtually insensitive to different orbitals used as a reference, even in the cases where quasi-degeneracy is present.