PARAMETRIZATION OF ALIPHATIC CHN UNITED ATOMS OF GROMOS96 FORCE-FIELD

The derivation of the van der Waals parameters for the aliphatic CHn u nited atoms of the GROMOS96 force field is presented. The parameters h ave been adjusted to reproduce the experimental enthalpies of vaporiza tion and vapor pressures or densities of a set of nine alkanes in the liquid state at 298 K (or at the boiling point in the case of methane) , using a cutoff radius for the van der Waals interactions of 1.6 nm. Force fields to be used in molecular simulations are bound to the cond itions chosen for their parametrization, for example, the temperature, the densities of the systems included in the calibration set, or the cutoff radius used for the nonbonded interactions. Van der Waals param eters for the CHn united atoms of earlier GROMOS force fields were dev eloped using a cutoff radius of 0.8 nm for the van der Waals interacti ons. Because the van der Waals interaction energy between aliphatic gr oups separated by distances between 0.8 and 1.4 nm is not negligible a t Liquid densities, the use of these parameters in combination with lo nger cutoffs leads to an overestimation of the attractive van der Waal s interaction energy. The relevance of this excess attraction depends on the size of the groups that are interacting, as well as on their lo cal densities. Free energies of hydration have been calculated for fiv e alkanes. (C) 1998 John Wiley & Sons, Inc.