## Entropy of a nonequilibrium system

Three definitions of entropy for a nonequilibrium system of particles, driven homogeneously by external forces and thermostatted homogeneously by a feedback mechanism, are discussed. The first is proposed to be S(t) = -k, i.e., the nonequilibrium ensemble average of the logarithm of the thermostatted equilibrium distribution function f/sub xi/. We show here, for a specific example, namely, the Nose-Hoover thermostat, that the entropy so defined reduces properly to the equilibrium result when the external forces are turned off, that this entropy behaves correctly when the thermostat is turned off, and that the thermostatted steady state is achievable. A reasonable alternativemore »