The effect of local approximations in coupled-cluster wave functions on dipole moments and static dipole polarisabilities

The influence of local approximations in electron correlation treatments on electric dipole moments and static dipole polarisabilities is examined for a test set of 16 molecules for the case of coupled-cluster singles and doubles (CCSD) theory. Utilising standard local approximations as originally proposed by Pulay and used in our previous work, the average errors relative to the corresponding conventional CCSD calculations amount to 0.48%, for mean dipole polarisabilities and to 1.61% for dipole moments. The accuracy of the computed mean dipole polarisabilities can be reduced to 0.32% by extending the domains of the strong pairs, while for the dipole moments the domain extension reduces the error to 0.91%. It is found that orbital relaxation effects are much more important in the local than in the conventional case. Weak pairs contribute substantially to the computed dipole polarisabilities, but the effect of distant pairs is small and can be neglected. This means that linear scaling local electron correlation methods can be faithfully used to compute these molecular properties.