On the choice of dihedral angle potential energy functions for n-alkanes

The parameters of the GROMOS96 force field governing dihedral angle transitions in aliphatic chains have been reconsidered, since these parameters produce a too large ratio of trans to gauche conformations in such chains. A refined set of parameters for dihedral angle interactions and third-neighbour interactions involving CH2 and CH3 atoms is proposed. They were obtained by fitting to the heat of vaporization, pressure and trans-gauche ratio for liquids of three n-alkanes, n-butane, n-pentane and n-hexane. The new parameter set does reproduce better these quantities and should therefore be more appropriate for use in simulations of polymers and membranes. A comparison of the mentioned properties obtained from simulations with united-atom models and from simulations with an all-atom model shows that the latter does not necessarily yield an improved description of molecular behaviour.