Investigation of the interaction between molecules at medium distances. III. SCF LCAO MO supermolecule, perturbational and MCF calculations for two and

Abstract

The interaction energies of 2HF, 2H 2O, 2CH 2O and 2HCONH 2 molecules at medium intermolecular distances have been calculated with the aid of the mutually consistent field and perturbational schemes. Instead of using the direct integration method the Coulomb potentials have been represented by a point charge distribution. The MCF model gives in all cases better results as compared to the supermolecule calculations than the perturbational ones. Also reported are the results for interaction energies of three systems consisting of three molecules 2HF + H 3O +, 2H 2O + H 3O + and 3H 2O, respectively, in the supermolecule, perturbational and MCF approach with the aid of potentials calculated by point charge distributions. The MCF results are in better agreement with the SM ones and show the influence of the third partner molecule on the interaction of the two other ones, while on the other hand this effect cannot be described in the perturbational scheme. Comparing the computer times needed in the three approaches, respectively, one can see that for small molecules using a medium size basis set the supermolecule calculations need less time than the two other schemes while for larger molecules ano even small molecules with an extended basis set the order is reversed and the MCF method needs less computer time than the perturbation theoretical scheme.