FIRST-ORDER ONE-ELECTRON PROPERTIES IN THE INTEGRAL-DIRECT COUPLED-CLUSTER SINGLES AND DOUBLES MODEL

An integral-direct implementation of first-order one-electron properti es in the coupled cluster singles and doubles (CCSD) model is presente d. The implementation increases the range of applicability of CCSD fir st-order one-electron property calculations significantly compared to nondirect approaches. As an application a thorough basis set investiga tion is performed on five diatomic molecules at the Hartree-Fock and C CSD levels for the molecular electric dipole moment, the molecular ele ctric quadrupole moment, and the electric field gradient at the nuclei . In general, basis sets of polarized triple-zeta quality are the smal lest to be recommended, and the convergence towards the basis set limi t is faster at the Hartree-Fock level than at the CCSD level. Among th e properties considered, the electric dipole moment is the easiest to converge. The electric dipole and especially the electric quadrupole m oment require diffuse functions for high accuracy. With standard basis sets, it is not possible to calculate electric field gradients consis tently within three thousandths of an atomic unit of the basis set lim it-for this purpose, elaborate nonstandard basis sets are required. Th e electric field gradients at the nuclei in HCN and the electric dipol e moment of the furan molecule are calculated at the CCSD level employ ing up to 417 basis functions, further demonstrating the large-scale a pplicability of the implementation. (C) 1997 American Institute of Phy sics.