Intermolecular interaction energies from minimal-basis SCF calculations. Interactions pertinent to formaldehyde hydration

The counterpoise method, for elimination of the basis-set superposition error in minimal-basis SCF MO calculations of intermolecular interactions, is extended to the general case involving geometrical distortion. Dispersion energies are estimated, using a modified London formula, as a sum of bond-bond terms involving bond polarisabilities and bond ionisation potentials taken from photoelectron spectral data. These methods are applied to the interactions of water and formaldehyde neutral molecules and to ion-molecule interactions of the hydroxide anion with formaldehyde and of water with protonated formaldehyde.