Nonlinear optical properties of the rhombic B₄ -cluster

The equilibrium geometry, energy, dipole polarizability, and second hyperpolarizability of the rhombic B₄ -cluster with D_2h symmetry are calculated by restricted (RHF) and unrestricted (UHF) Hartree-Fock theory; Møller-Plesset 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 state wave function. Accordingly, the RHF method yields results that differ greatly from those of the correlated methods, even for the geometry. UHF gives more reliable geometries, but suffers from high spin contamination. The electric properties calculated at reasonably highly correlated levels are qualitatively comparable for both the single reference and multireference descriptions, although differences between CCSD(T) and BD(T) are larger than usually reported in the literature and properties calculated at the MPn (n=2, 3, 4) series show much damped oscillatory behavior, especially for the components along the long in-plane axis. It is found that inclusion of f-functions in the basis set do not have a large effect on the electric properties. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 679-687, 1999.