Nonlinear optical properties of the rhombic B-4-cluster

The equilibrium geometry, energy, dipole polarizability, and second hyperpo larizability of the rhombic B-4-cluster with D-2h symmetry are calculated b y restricted (RHF) and unrestricted (UHF) Hartree-Fock theory; Moller-Pless et perturbation theory (MPn); coupled-cluster theory with singles, doubles, and a perturbational treatment for connected triples [CCSD(T)]; Brueckner doubles with a perturbational treatment for connected triples [BD(T)]; and complete active space SCF (CASSCF) and restricted active space SCF (RASSCF) methods, using three different basis sets. The multireference methods show that excited state configurations contribute appreciably to the ground sta te wave function. Accordingly, the RHF method yields results that differ gr eatly from those of the correlated methods, even for the geometry. UHF give s more reliable geometries, but suffers from high spin contamination. The e lectric properties calculated at reasonably highly correlated levels are qu alitatively comparable for both the single reference and multireference des criptions, although differences between CCSD(T) and BD(T) are larger than u sually reported in the Literature and properties calculated at the MPn (n = 2,3,4) series show much damped oscillatory behavior, especially for the co mponents along the long in-plane axis. It is found that inclusion of f-func tions in the basis set do not have a large effect on the electric propertie s. (C) 1999 John Wiley & Sons, Inc. J Comput Chem 20: 679-687, 1999.