COMPARISON OF LANGEVIN AND MOLECULAR-DYNAMICS SIMULATIONS - EQUILIBRIUM AND DYNAMICS OF ETHYLENE-GLYCOL IN WATER

The results of molecular dynamics (MD) simulations of one ethylene glycol molecule in 259 waters from trajectories totalling 5 ns are compared with those from Langevin dynamics simulations of a single ethylene glycol. It is found that while the trans-gauche equilibrium constant is relatively unperturbed by water, the effective barrier for isomerization is lowered by ca. 1 kT. Once this effect is taken into account, a Langevin model without explicit waters with a collision frequency, gamma, of ca. 50 ps-1 provides a reasonable description of the average isomerization rate. Overall rotation is well fitted with gamma = 75 ps-1, although the fast librational motion present in MD is overdamped in LD. When hydrodynamic interaction is introduced with the Oseen tensor and the above collision frequencies are assumed, 1-2 bound waters are required to reproduce the translational diffusion constant of the glycol obtained in the MD simulations. These results indicate that Langevin dynamics simulations can reproduce some behaviour of polar molecules in aqueous solutions, but might be better carried out including a partial layer of bound water.