A non-empirical intermolecular potential for oxalic acid crystal structures

A model intermolecular potential for oxalic acid is derived from the properties of the isolated molecule. Various methods of obtaining an isotropic atom-atom repulsion potential from the overlap of the monomer charge distributions are investigated. These model repulsion potentials are combined with an electrostatic model based on a distributed multipole analysis of the monomer wave function and a Slater-Kirkwood dispersion model using atomic polarizabilities. The resulting model potentials are tested for their ability to reproduce the two crystal forms of oxalic acid. The best models do reasonably well, within the limitations of static lattice energy minimization of rigid molecules. Since current transferable empirical model potentials used for modeling carboxylic acid crystal structures have problems in accounting for the oxalic acid polymorphs, this success shows the benefits of deriving specific model atom-atom potentials for organic molecules without relying on transferability assumptions.