Transferable ab Initio Intermolecular Potentials. 1. Derivation from Methanol Dimer and Trimer Calculations

An intermolecular potential was derived from ab initio calculations on methanol dimers and trimers. Ninety-four methanol dimer geometries and seventeen trimer geometries have been studied using an interaction-optimized basis set. Because we aimed at transferability, all terms of the model were fitted separately, in
order to obtain a potential in which these energy terms have a well-defined physical meaning. Electrostatic
interactions were described by atomic multipole moments obtained by fitting to the monomer electrostatic
potential. The polarization energy was modeled using atomic dipole polarizabilities. Scaled empirical
polarizabilities reproduced the energy nonadditivity in methanol trimers very accurately. The dispersion energy
was described by a damped r-6 atom−atom potential, which was fitted to separately calculated dispersion
energies. Exchange energy and remaining short-ranged terms were modeled by an exponential repulsion term,
including some anisotropic features. This repulsion model was fitted to the total SCF+MP2 interaction energies
of the dimers. The maximum deviation for near-equilibrium geometries was ∼0.2 kcal/mol. The calculated
interaction energies for dimers containing methane, water, and dimethyl ether were in similar agreement,
both with our own ab initio calculations and with the best values available in the literature. In addition,
trends in hydrogen-bond distances in these dimers and in methanol trimers were well reproduced by our
model. So, the potential was seen to be transferable to related systems.