Development of an intermolecular potential function for interactions in formamide clusters based on ab initio calculations

A potential function for describing interactions between formamide molecules is proposed. The function was developed from ab initio computed molecular properties and intermolecular perturbation theory (IMPT) calculations for formamide dimer involving the 6-311G** basis set. It consists of an electrostatic term that is a function of multipoles distributed over the atoms, an exponential repulsion term obtained by fitting results for the dimer, an induction contribution that is a function of atomic polarizabilities, and a dispersion term based on a London expression that is also dependent on atomic polarizabilities. The results obtained by applying the proposed function to formamide dimer are similar to those found at the MP2/6-311G** level; the function allows one to identify five structures corresponding to as many energy minima. Application of the function to larger clusters revealed that the most attractive minima correspond to planar structures, the most common structural pattern among which is that of the global minimum for the dimer. Based on cooperativeness data for the larger clusters, chained structures seemingly form stronger hydrogen bonds due to increased cooperativeness in interactions between molecules, which may account for the tendency of condensed phases of formamide to adopt open structures. (C) 1999 American Institute of Physics.