Improved Effective-Potential Monte Carlo Theory

The free energy of a quantum crystal with large zero-point motion, calculated using the effective-potential Monte Carlo (EPMC) method, is deficient at zero temperature because of the omission of cubic terms in the potential. We propose an improved effective-potential theory which includes a perturbative cubic correction. From the many possible forms of this correction, consistency requirements indicate a unique one. We show that, for a model of neon, this correction leads to superior results, and that the EPMC method's speed and ease of computation are preserved.