Improved partition-expansion of two-center distributions involving slater functions

The calculation of the electronic structure of large systems is facilitated by the substitution of the two‐center distributions by their projections on auxiliary basis sets of one‐center functions. An alternative is the partition–expansion method in which one first decides what part of the distribution is assigned to each center, and next expands each part in spherical harmonics times radial factors. The method is exact, requires neither auxiliary basis sets nor projections, and can be applied to Gaussian and Slater basis sets. Two improvements in the partition–expansion method for Slater functions are reported: general expressions valid for arbitrary quantum numbers are derived and the efficiency of the procedure is increased giving analytical solutions to integrals previously computed by numerical quadrature. The efficiency of the new version is assessed in several molecules and the advantages over the projection methods are pointed out. © 2013 Wiley Periodicals, Inc. The calculation of the electronic structure of large systems is facilitated by expanding the two‐center distributions in terms of one‐center functions. The partition–expansion method is an alternative to the standard projection methods which yields a systematic procedure for improving the fit of two‐center distributions in pairs of one‐center expansions. The method does not require auxiliary basis sets for projection and it allows to attain accurate expansions at a reduced cost.