TRANSFERABLE NET ATOMIC CHARGES FROM A DISTRIBUTED MULTIPOLE ANALYSISFOR THE DESCRIPTION OF ELECTROSTATIC PROPERTIES - A CASE-STUDY OF SATURATED-HYDROCARBONS

Point charges derived from electrostatic potentials and from distribut ed multipole expansions are compared critically throughout a full ab i nitio self-consistent-field study of several saturated hydrocarbons, u sing the split-valence type 6-31G, 6-31G, and 6-31G** basis sets. Pot ential-derived net atomic charges failed to be transferable from one m olecule to another structurally similar one, especially for aliphatic groups, hence constituting a major drawback for their use in force-fie ld parametrization. Consideration of a limited number of extra-atomic charges is necessary to reach transferability and mimic the effects of higher order moments of the electron charge distribution. However, it has been shown that another method, recently proposed and based on th e distributed multipole expansion of the molecular electrostatic poten tial, provides a reasonable compromise for both chemical transferabili ty and accuracy to reproduce the electrostatic potential. Nevertheless , charges obtained from both methods have a tendency to reproduce the SCF potential with conspicuous errors, therefore indicating that the m olecular charge distribution of nonpolar systems cannot be represented accurately by only atom-centered point charge models.