Ab initio study of the electric-field-gradient-induced birefringence of a polar molecule: CO

An ab initio coupled cluster singles and doubles (CCSD) investigation of th e electric-field- gradient-induced birefringence (EFGB) of a polar molecule , CO, is presented. The so-called "effective quadrupole center" (EQC), the origin to which the quadrupole moment deduced from EFGB experiments refers for dipolar molecules and which experimentalists cannot determine directly, is computed within two different semiclassical theories of the EFGB. The t emperature independent frequency dependent contribution to the birefringenc e is determined as well. The molecular electric quadrupole moment with the origin at the EQC is determined within the two theories via an accurate val ue of the quadrupole moment referring to the center of mass, obtained using well-established hierarchies of basis sets and wave function models, and t he calculated EQCs. The final theoretical values result as the basis set li mit at the CCSD(T) level completed with estimates for relativistic effects, correlation contributions beyond CCSD(T), rovibrational effects, and resid ual errors in the EQC. The comparison of the theoretical results with the c orresponding experimentally deduced values sheds some light on the differen ces between the two semiclassical approaches to the EFGB. (C) 2000 American Institute of Physics. [S0021-9606(00)30332-4].