Intermolecular interactions in crystals of carboxylic acids: III. Non-empirical interatomic potential functions

Atom-atom potentials for carboxylic acids have been derived by making a least-squares fit to interaction energies, calculated by the ab initio MO-SCF method. Energies for 12 configurations of the formic acid dimer and 16 configurations of the methanol/formaldehyde complex were included in the process. Using (exp-6-1) atom-centred potentials, augmented with attractive exponential functions centred on the hydroxyl hydrogen and carbonyl oxygen atoms a satisfactory fit is obtained (i.e. agreement within 0·8 kcal mol -1 ), especially for the hydrogen-bonded configurations of the formic acid pair. The resulting potential set is applied to the calculation of the structures and energies of the formic acid dimer and crystal, the acetic acid dimer and crystal and the two modifications of crystalline oxalic acid. The structures of these systems which result from the minimization of the dimer or lattice energies with respect to structural parameters show good agreement with experiment and are almost as accurate as the structures obtained with empirical potential sets. The calculated energies are in error by 5-8 kcal mol -1 , which is due to systematic errors in the SCF method, to the use of small basis sets and to transfer errors.