Coupled cluster calculations of the electric properties of BeS. An analysis of the fifth-order non-iterative corrections

The static dipole and quadrupole moments, as well as various higher polarizability tensors, are calculated for the BeS molecule using various coupled cluster (CC) variants with the inclusion of single and double excitation operators (CCSD) and an a posteriori approximate treatment of the triples (CCSD(T)). Moreover, full inclusion of the singles, doubles and triples (CCSDT) is employed for dipole moment and dipole polarizability calculations, for which also the fifth-order non-iterative corrections are analysed in detail. The important role of electron correlation effects is stressed. A comparison of values calculated as energy derivatives with values obtained as expectation values and/or expectation value derivatives is made. This comparison confirms that as soon as the triples are included both methods provide similarly reliable results.