A portable intermolecular potential for molecular dynamics studies of NMA-NMA and NMA-H2O aggregates.

A recently formulated intermolecular potential has been adapted to describe the interaction of the N-methylacetamide (NMA) dimer and of the NMA-H(2)O adduct. The pure electrostatic component of the intermolecular potential functional representation is as usual expressed in terms of a set of punctual charges distributed over the molecular frames, consistently with the permanent molecular dipole values. In contrast, the remainder of the intermolecular potential is expressed in terms of Improved Lennard Jones effective pair potential functions, referred to multiple interaction centers (or sites) placed on the N-methylacetamide molecule and to a single interaction center placed on the water molecule. The characteristic of this pair potential relies on a mix of transferable and non-transferable descriptions of the parameters. The first set of parameters has a structural connotation bearing a site-site interaction nature and exploiting the molecular polarizability decomposability. The second one, depending on the particles clustering and charge distribution and transfer, bears a delocalized and ambient bulk nature. This choice has been tested against ab initio calculations and molecular dynamics simulations. The results show that the model potential is appropriate for describing the energetic of the various stable configurations of NMA-NMA and NMA-H(2)O weakly interacting aggregates, including the formation of hydrogen bonds.