CUTOFF EFFECT IN MOLECULAR-DYNAMICS SIMULATIONS OF INTERACTION-INDUCED LIGHT-SCATTERING SPECTRA

A strong dependence of the interaction induced light scattering integr ated 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 concl uded that the uncertainties arising from this effect were much larger than can be tolerated in any evaluation of the light scattering mechan ism. No real explanation or solution ever appeared in the literature o n this problem. Here we report a very strong dependence of the interac tion induced spectra calculated by molecular dynamics simulations on t he number of atoms used in a liquid argon simulation. We show that thi s 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 cuto ff radius R-cutoff. To obtain accurate spectra up to high frequency sh ifts, large cutoff radii have then to be used corresponding to thousan ds of atoms in the simulation. The time of the simulation is then proh ibited. An explanation for both effects is proposed and two different methods to solve this problem are tested. (C) 1997 Elsevier Science B. V.