Comparison of MBPT and coupled cluster methods with full CI. II. Polarized basis sets

Abstract

Results from many-body perturbation theory (MBPT) and coupled-cluster (CC) calculations relative to a single reference function are compared with recent full CI results for Ne, F, F−, HF, H2O, and NH2. MBPT results include second through fourth order, while CC results are reported for the CCSD, CCSD+T(CCSD), and CCSDT-1 methods. The basis sets used are of double-zeta plus polarization quality or better. HF, H2O, and NH2 are studied at several displaced geometries. Agreement to within 0.3 kcal/mol is found between infinite order CCSDT-1 results and full CI at the equilibrium geometries, while finite-order MBPT(4) is still quite good. For FH, CCSDT-1 relative to an incorrectly separating RHF reference is still accurately described all the way to the separated atom limit. At 2.0 Re, only the CCSDT-1 method obtains satisfactory agreement with full CI, having an average error of 3.6 kcal/mol, while MBPT(4) has an average error of 13.1 kcal/mol.