COUPLED-CLUSTER CALCULATIONS OF THE ELECTRIC PROPERTIES OF BES - AN ANALYSIS OF THE 5TH-ORDER NONITERATIVE CORRECTIONS

The static dipole and quadrupole moments, as well as various higher po larizability tensors, are calculated for the BeS molecule using variou s coupled cluster (CC) variants with the inclusion of single and doubl e excitation operators (CCSD) and an a posteriori approximate treatmen t of the triples (CCSD(T)). Moreover, full inclusion of the singles, d oubles and triples (CCSDT) is employed for dipole moment and dipole po larizability calculations, for which also the fifth-order non-iterativ e corrections are analysed in detail. The important role of electron c orrelation effects is stressed. A comparison of values calculated as e nergy derivatives with values obtained as expectation values and/or ex pectation value derivatives is made. This comparison confirms that as soon as the triples are included both methods provide similarly reliab le results.