An efficient treatment of kinematic factors in pseudo-relativistic calculations of electronic structure

Relativistic two-electron operators obtained by the Douglas-Kroll transformation contain p-dependent kinematic prefactors which are,not present in the corresponding Breit-Pauli operators. These factors are usually calculated using a resolution of the identity approach, which requires integral transformations. In the present article we describe an alternative approach, based on a local approximation, where the effect of the prefactors can; be absorbed in the atomic contraction coefficients. The effect of the local approximation is investigated in detail. The suggested approach is simple to implement in integral codes that allow the use of a general contraction scheme.