Molecular dynamics studies on potential energy fluctuations of particles in Lennard-Jones systems

Molecular dynamics (MD) calculations were performed for a Lennard-Jones fluid system over a wide density range. Pair correlation functions and potential energy fluctuations and to a lesser extent potential energy distributions have been determined. The analysis of potential energy of the system was done in terms of the mean standard deviations of the repulsive, attractive and total contributions separately. The results are as follows: (i) the range of validity of first order perturbation approaches such as that of Weeks, Chandler and Andersen (WCA) can be predicted quantitatively; (ii) the frequently used picture of a uniform ‘background potential’ is precisely supported for a surprisingly large density range; (iii) simple, physically plausible extensions of the WCA approach for a larger density range could not be extracted; (iv) an ad hoc correction of the WCA treatment predicts however the pair correlation functions in agreement with MD-results for a reduced density range of 0·15 ⩽ ρ* ⩽ 0·65 to better than 6 per cent at temperatures of 120–200 K. The correction term is easy to compute.