On the algorithm to perform Monte Carlo simulations in cells with constant volume and variable shape

In simulations of crystals, unlike liquids or gases, it may happen that the properties of the studied system depend not only on the volume of the simulation cell but also on its shape. For such cases it is desirable to change the shape of the box on the fly in the course of the simulation as it may not be known ahead of time which geometry fits the studied system best. In this work we derive an algorithm for this task based on the condition that the distribution of specific geometrical parameter observed in simulations at a constant volume matches that observed in the constant-pressure ensemble. The proposed algorithm is tested for the system of hard-core ellipses which makes lattices of different types depending on the asphericity parameter of the particle. It is shown that the performance of the algorithm critically depends on the range of the sampled geometrical parameter. If the range is narrow, the impact of the sampling method is minimal. If the range is large, inadequate sampling can lead to significant distortions of the relevant distribution functions and, as a consequence, errors in the estimates of free energy.