Multipole approximation of distant pair energies in local MP2 calculations
by Hetzer, G.; Pulay, P.; Werner, H. J.
An approximation for treating the weak correlation of distant pairs in local MP2 calculations by multipole expansions is proposed. This leads to computational savings in the transformation of the two-electron integrals, which is the bottleneck in MP2 calculations. The method works well for orbital pairs that are at least 7 bohr apart. The accuracy is mainly limited by the neglect of ionic excitations in the first-order wavefunction, while the error of the multipole expansion is almost negligible.