MOLECULAR QUADRUPOLE-MOMENT FUNCTIONS OF HF AND N-2 .1. AB-INITIO LINEAR-RESPONSE COUPLED-CLUSTER RESULTS

The orthogonally spin-adapted linear-response coupled-cluster (LRCC) t heory with singly and doubly excited clusters (CCSD) was employed to c alculate quadrupole moment functions of the HF and N-2 molecules in th eir ground electronic states. We also calculated several potential ene rgy curves for both systems using various CC and non-CC methods, rangi ng from the limited and full configuration interaction (CI) and first- order CI (FOCI) to finite-order many-body perturbation theory. FOCI an d related complete active space self-consistent field (CASSCF) methods were used in both energy and quadrupole moment calculations. Most of the calculations were performed using the medium-size basis set of TZ2P quality devised by Sadlej [A. Sadlej, Cell. Czech. Chem. Commun. 53 , 1995 (1988)] for high-level ab initio calculations of electrostatic molecular properties. In addition, a number of model CC calculations u sing small basis sets were performed, for which the exact full CI resu lts, both for the energy and multipole moments, are available. It was demonstrated that the CCSD approach provides a realistic description o f quadrupole moment functions, for all relevant geometries in the case of HF and for internuclear separations up to 1.5 times the equilibriu m bond length for N-2. The results of this study will be used for the analysis of the rovibrational dependence of quadrupole moments and for the Calculation of quadrupole transition moments for both HF and N-2. (C) 1996 American Institute of Physics.