Intermolecular potentials and force constants from ab initio energies - Application to the N-H...O=C hydrogen bonds in formamide dimers

The ab intitio energies and force constants of 38 geometrically optimized formamide dimers which differ in the lengths of the hydrogen bonds, are evaluated using the program GAUSSIAN 90 with the 6-31G** basis set. A potential energy function was fitted simultaneously to the dimerization energies (including vibrational energy contributions to association energies) and the force constants of the N-H ... O=C bridge. As an application, the broadening of the signals in vibrational spectra of liquid formamide was simulated by a superposition of spectra of different formamide oligomers.