Beyond Gaussian fluctuations of quantum anharmonic nuclei

Gaussian fluctuations describe the quantum state of the nuclei, even in strongly anharmonic materials. However, this approximation fails spectacularly in molecular crystals with rotational motions and in tunneling modes. This paper presents a variational approach that overcomes these limitations through a nonlinear transformation of Cartesian coordinates. Employing Monte Carlo sampling of the quantum phase space at finite temperatures, the method maintains an analytical expression for the entropy, which ensures direct access to the free energy and, consequently, the phase diagram of materials.