Intermolecular interactions in crystals of carboxylic acids

The dimerization and lattice energies of formic acid are calculated using the non-empirical MO-SCF method. The interaction energies are partitioned into five components that are related directly to monomer wavefunctions and to their properties. The lattice energy is calculated in the pair approximation, and it is shown that only seven molecular pairs have to be treated at the SCF level, since the remaining interactions in the crystal can be adequately described by multipole expansions. The electrostatic multipole energy is corrected for errors in the monomer SCF multipole moments and the dispersion energy is scaled to compensate for the use of truncated basis sets. The corrected dimer and lattice energy (-14 and -13 kcal mol-1) agree well with experiment (-16·0 and -14·8 kcal mol-1). Likewise the equilibrium separation in the dimer is reproduced to within 0·04 Å. There is, however, a large uncertainty in the calculated energies owing in part to the unknown effect of electron correlation upon the first order exchange energy and in part to the use of truncated basis sets. Non-additivity effects in the crystal are neglected in this approach but their contributions are estimated to be less than 1 kcal mol-1.