Single reference coupled cluster calculations for weakly bound alkaline-earth metal dimers in the ground state: a useful perturbative scheme for an iterative triples correction

Abstract

We apply here a new iterative triples correction for the single reference coupled-cluster method with singles and doubles scheme (termed as CCSDT-1a+d). All diagonal terms in have been included in the T 3 determining equation, in addition to . This minor change in the CCSDT-1a approximation (includes the primary effects of connected triples excitations iteratively) shows a dramatic change in the study of correlation of weakly bonded systems as discussed here. To achieve a general assessment of the potentiality of the CCSDT-1a + d calculations, the ground state potential energy surfaces (PESs) of alkaline-earth dimers, including Mg2, Ca2 and the challenging Be2 are considered. The systems considered here are treated as four-electron systems, keeping other electrons frozen in the correlation calculations. Generation of PESs for these dimers is very sensitive to the level of theory employed for the electron correlation. Basic spectroscopic constants including dissociation energy extracted from the calculated ground state PES are also reported. For all systems under study, the overall performance of CCSDT-1a+d is very competitive with that of the CCSD(T)/CCSDT method, and better than that of CCSDT-1a indicating that the former approach is more effective in handling dynamic correlation effects in comparison to the latter one in the presence of quasi-degeneracy to different extents. To calibrate our results we also report frozen-core FCI calculations.