An accurate determination of rovibrational spectra using the externally corrected coupled-cluster approaches: LiH ground state

Recently acquired highly precise spectroscopic data for the ground state of LiH and its various isotopomers are employed to carry out a critical assessment of the performance of the externally corrected coupled-cluster (CC) approaches. Both the amplitude and energy-corrected approaches are considered, in particular the reduced multireference CC method with singles and doubles (RMR CCSD) and the asymmetric energy formula based CCSD-[MR] method, both exploiting the same modest-size multireference configuration interaction (MR CISD) wave function, based on an M-dimensional reference space, as the source of higher than pair clusters. To assess the size of the basis set errors relative to those of the methods employed, the comparison is also made with the full CI (FCI) results at the cc-pVTZ level. The rovibrational energy levels and the corresponding transition frequencies are then computed for various isotopomers of LiH using the theoretically determined potentials at the cc-pVXZ (X=D, T, Q, and 5) and the extrapolated completebasis-set-limit levels and a comparison is made with the experiment as well as with the existing theoretical results, particularly those exploiting the CC approaches. The role of adiabatic corrections is also assessed. It is shown that both the RMR CCSD and CCSD-[4R] methods, yielding an almost indistinguishable results, are capable of accounting for the nondynamical correlation effects that are lacking in the standard single-reference CCSD approaches. (C) 2003 American Institute of Physics.