A systematic nonempirical method of deriving model intermolecular potentials for organic molecules: Application to amides

A systematic method of deriving atom-atom intermolecular potentials from th e monomer wave functions has been developed for formamide, acetamide and tl trans-N-methylacetanlide (NMA) and tested for its ability to reproduce the crystal structures. The total intermolecular potentials comprised an accur ate distributed multipole analysis representation of the multipolar electro static interaction energy, an atom-atom Cg dispersion model, and a short-ra nge repulsion model derived from the overlap of the monomer charge densitie s. The short-range model has been assessed and validated by comparison with ab initio intermolecular perturbation theory (IMPT) calculations of the ex change-repulsion, penetration, and charge-transfer energies of test sets of around 20 dimer conformations. A range of models has been developed. The s implest version of the overlap model need not require any IMPT calculations (though in this example they are used to calibrate and validate the potent ial) and can be used to estimate atom-atom repulsive parameters. Removal of various simplifying assumptions in the overlap model gives better reproduc tions of the IMPT data and the crystal structures, and provides a route to specific potentials for organic molecules. The resulting model potentials, as assessed by crystal structure reproduction, are comparable with the best empirical potentials for amides and superior to some commonly used potenti al energy functions. An advantage of the method is that transferability of parameters can be tested, rather than assumed. There is an encouraging degr ee of transferability as the potential generated for NMA reproduces the cry stal structures of formamide and bothpolymorphs of acetamide very well.