MOLECULAR-DYNAMICS FREE-ENERGY CALCULATION IN 4 DIMENSIONS

A new method for the calculation of free energy differences by molecul ar dynamics simulations of molecular systems is presented. The main ch aracteristic of the method is that the three-dimensional physical spac e is augmented by a further dimension. This allows the system to circu mvent energy barriers in three-dimensional space. When simulating phys ical (three-dimensional) states, the dynamics along the 4th coordinate is uncoupled from the dynamics described with the original three coor dinates. In the method presented here, free energy differences between physical states are determined using pathways leading over states whe re the dynamics along all four dimensions are coupled. Due to the barr ier lowering in four dimensions, an increase of efficiency can be expe cted when low free energy physical states are separated by high barrie rs. The method is tested by application to an atomic liquid.