Cutoff effect in molecular dynamics simulations of interaction induced light scattering spectra

A strong dependence of the interaction induced light scattering integrated intensity with the cutoff radius was previously reported in noble liquids molecular dynamics simulations (A.J.C. Ladd, T.A. Litovitz, J.H.R, Clarke, L.V. Woodcock, J. Chem. Phys. 72 (1980) 1759; and J.H.R. Clarke, L.V. Woodcock, Chem. Phys. Lett. 78 (1981) 121). It was concluded that the uncertainties arising from this effect were much larger than can be tolerated in any evaluation of the light scattering mechanism. No real explanation or solution ever appeared in the literature on this problem. Here we report a very strong dependence of the interaction induced spectra calculated by molecular dynamics simulations on the number of atoms used in a liquid argon simulation. We show that this effect is proportional to the square of the density involved in the simulation and that it obeys to a R-cutoff(-4) power law with the cutoff radius R-cutoff. To obtain accurate spectra up to high frequency shifts, large cutoff radii have then to be used corresponding to thousands of atoms in the simulation. The time of the simulation is then prohibited. An explanation for both effects is proposed and two different methods to solve this problem are tested. (C) 1997 Elsevier Science B.V.