COMPARISON OF MULTIPOLE MOMENT EXPANSIONS BY DIRECT SUMMATION OR FINITE-FIELD SCF METHODS WITH FULL ELECTROSTATIC INTERACTION ENERGIES - APPLICATION TO CO AND N2

In calculations of the electrostatic interactions of molecules with external potentials the multipole expansion is frequently employed. In this paper the usefulness and validity of this method are investigated. Two types of potential, that due to a point charge and that near the (100) surface of a NaCl crystal have been considered, and numerical results are presented for CO and N2. Multipole expansions up to the hexadecapole level have been calculated both by the summation of moment contributions and by a finite-field SCF approach. Problems in applying the expansion technique are discussed. An alternative method for calculating interactions with potentials of arbitrary functional form is proposed. A possible explanation of the differences in physisorption of CO and N2 is advanced. A comparison of electrostatic potential curves for N2 using an extended basis set with those published for a minimal basis set are given and important differences are examined. © 1978.