Monte Carlo computations of the quantum kinetic energy of rare-gas solids

We report results from Monte Carlo computations for the average kinetic energy of rare-gas solids (neon, argon, krypton, and xenon), modeled by a Lennard-Jones all-neighbor interaction. The main motivation lies in the recent availability of direct experimental measurements of the average kinetic energy of solid neon, by means of deep-inelastic neutron scattering (DINS). In our computations we take strong advantage in using the effective potential technique, which has been proven to be very useful for systems where quantum effects are not too strong: the path-integral Monte Carlo (PIMC) can be replaced by the classical-like effective-potential Monte Carlo (EPMC), in such a way that the needed computer time is strongly reduced. We resorted to PIMC in the case of neon, due to its rather high quantum effects. Our results for the low-temperature kinetic energy of neon are smaller than the measured ones. This discrepancy could be attributed to the simple model of the interaction we have used, as the agreement with previous theoretical calculations suggests. Moreover, we show that the quantum contributions to the kinetic energy, at the same temperatures used in the above-mentioned experiments, are unexpectedly relevant also for argon, krypton, and xenon crystals, so that they should be experimentally detectable as well.