INTER-MOLECULAR INTERACTIONS IN CRYSTALS OF CARBOXYLIC-ACIDS .3. NON-EMPIRICAL INTER-ATOMIC 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 t systems which result from the minimization of the dimer or lattice energies respect to structural parameters show good agreement with experiment are almost as accurate as the structures obtained with empirical potei sets. The calculated energies are in error by 5-8 kcal mol-1, which is Ax systematic errors in the SCF method, to the use of small basis sets an transfer errors. © 1979 Taylor & Francis Group, LLC.