Comparison of thermostatting mechanisms in NVT and NPT simulations of decane under shear

Nonequilibrium molecular dynamics (NEMD) simulations play a major role in c haracterizing the rheological properties of fluids undergoing shear flow. H owever, all previous studies of flows in molecular fluids either use an "at omic" thermostat which makes incorrect assumptions concerning the streaming velocity of atoms within their constituent molecules, or they employ a cen ter of mass kinetic (COM) thermostat which only controls the temperature of relatively few degrees of freedom (3) in complex high molecular weight com pounds. In the present paper we show how recently developed configurational expressions for the thermodynamic temperature can be used to develop therm ostatting mechanisms which avoid both of these problems. We propose a therm ostat based on a configurational expression for the temperature and apply i t to NEMD simulations of decane undergoing Couette flow at constant volume and at constant pressure. The results so obtained are compared with those o btained using a COM kinetic thermostat. At equilibrium the properties of sy stems thermostatted in the two different ways are of course equivalent. How ever, we show that the two responses differ far from equilibrium. In partic ular, we show that the increase in the potential energy of the internal mod es with increasing shear is only observed with a Gaussian isokinetic COM th ermostat in both NVT and NPT simulations. There is no such increase with th e configurational thermostat, which, unlike the Gaussian isokinetic COM the rmostat, correctly accounts for the internal degrees of freedom of the mole cular fluid. (C) 2001 American Institute of Physics.