A GENERAL PRESSURE TENSOR CALCULATION FOR MOLECULAR-DYNAMICS SIMULATIONS

A hybrid method for the calculation of the pressure tenser in molecula r dynamics (MD) simulations, which combines both the thermodynamic and mechanical definitions, is presented here. The need for a new approac h was motivated initially by MD simulations of crystalline poly(ethyle ne oxide) (PEO) (S. Neyertz, D. Brown, and J. O. Thomas, 1994, J, chem . Phys., 101, 10064), as the combination in this model of effectively 'infinite' chains, holonomic constraints and the long-range Coulomb po tential calculated via the Ewald summation method, presented difficult ies with regard to the traditional routes for calculation of the press ure tenser. However, this technique is quite general and thus applicab le also to most MD simulations performed using periodic boundary condi tions. In this paper, we address in detail the contribution of forces arising from different parts of the potential and the constraints to t he pressure tenser. The method is then illustrated in a model of cryst alline PEO, where the sensitivity of the pressure tenser to details of the potential provides an effective criterion for assessing possible parameter sets.