A Boltzmann generator for the isobaric-isothermal ensemble

Abstract: Boltzmann generators (BGs) are now recognized as forefront generative models for sampling equilibrium states of many-body systems in the canonical ensemble, as well as for calculating the corresponding Helmholtz free energy. Furthermore, BGs can potentially provide a notable improvement in efficiency compared to conventional techniques such as molecular dynamics (MD) and Monte Carlo (MC) methods. By sampling from a clustered latent space, BGs can circumvent free-energy barriers and overcome the rare-event problem. However, one major limitation of BGs is their inability to sample across phase transitions between ordered phases. This is due to the fact that new phases may not be commensurate with the box dimensions, which remain fixed in the canonical ensemble. In this work, we present a novel BG model for the isothermal-isobaric (NPT) ensemble, which can successfully overcome this limitation. This unsupervised machine-learning model can sample equilibrium states at various pressures, as well as pressure-driven phase transitions. We demonstrate that the samples generated by this model are in good agreement with those obtained through MD simulations of two model systems. Additionally, we derive an estimate of the Gibbs free energy using samples generated by the NPT BG.