Multipole moments from the partition–expansion method

Analytical expressions for the atomic multipole moments defined from the partition–expansion method are reported for both Gaussian and Slater basis sets. In case of Gaussian functions, two algorithms are presented and examined. The first one gives expressions in terms of generalized overlap integrals whose master formulas are derived here with the aid of the shift-operator technique. The second uses translation methods, which lead to integrals involving Gaussian and Bessel functions, which are also known. For Slater basis sets, an algorithm based on translation methods is reported. In this algorithm, atomic multipoles are expressed in terms of integrals involving Macdonald functions, which have been solved in previous works. The accuracy of these procedures is tested and their efficiency illustrated with practical applications, including the computation of the full molecular electrostatic potential (not only the long-range) in large systems.