On the electric field gradient induced birefringence and electric quadrupole moment of CO, N2O and OCS.

An ab initio coupled cluster investigation of the electric field gradient induced birefringence of three dipolar linear molecules --- CO, N$_2$O and OCS --- is presented. Special emphasis is given to the effect of triple excitations in the cluster operator on the so-called effective quadrupole center as well as to the accurate determination of the molecular electric quadrupole moment relative to this origin. Triple excitations are found to be essential in the case of CO, with reference in particular to the existing disagreement between two semiclassical theories for the interpretation of the experimentally observed birefringence. The present results favor the older theory due to Buckingham and Longuet-Higgins.