AMYR 2: A new version of a computer program for pair potential calculation of molecular associations

Abstract

AMYR is a computer program for the calculation of molecular associations using Fraga's pairwise atom-atom potential. The interaction energy is evaluated through a 1 R expansion. The electrostatic energy is calculated through either the one-centre-per atom or the three-centres-per atom model by Hunter and Sanders. A pairwise dispersion energy term is included in the potential and corrected by a damping function. The program carries out energy minimizations through variable metric methods. The new version allows for the stationary point analysis of the intermolecular potential by means of the Hessian eigenvalues. Although using low-gradient thresholds optimization procedures to avoid many stationary points that are not true minima, computing time considerations makes the usual procedure of using high-gradient thresholds (say, 10 −2 kJ mol −1Å −1) as the most efficient. Moreover, this latter procedure can be recommended because the actual minima can be characterized by means of the Hessian eigenvalues even if these high-gradient thresholds are used, and further decreasing of the convergence criterion does not imply significant modifications in the geometric parameters of the minima. The program also calculates geometric descriptors and topological indices of the molecules and dimmer.