Direct reaction field force field: A consistent way to connect and combine quantum-chemical and classical descriptions of molecules

The direct reaction field (DRF) force field gives a classical description of intermolecular interactions based on ab initio quantum-chemical descriptions of matter. The parameters of the DRF force field model molecular electrostatic and response properties, which are represented by distributed charges and dipole polarizabilities. The advantage of the DRF force field is that it can be combined transparently with quantum-chemical descriptions of a part of a large system, such as a molecule in solution or an active site in a protein. In this study, the theoretical basis for the derivation of the parameters is reviewed, paying special attention to the four interaction components: electrostatic, induction, dispersion, and repulsion. The ability of the force field to provide reliable intermolecular interactions is assessed, both in its mixed quantum-chemical-classical and fully classical usage. Specifically, the description of the water dimer and the solvation of water in water is scrutinized and seen to perform well. The force field is also applied to systems of a very different nature, viz. the benzene dimer and substituted-benzene dimers, as well as the acetonitrile and tetrachloromethane dimers. Finally, the solvation of a number of polar solutes in water is investigated. It is found that as far as the interaction energy is concerned, the DRF force field provides a reliable embedding scheme for molecular environments. The calculation of thermodynamic properties, such as solvation energy, requires better sampling of phase space than applied here. (C) 1996 John Wiley & Sons, Inc.