Finite-size scaling for near-critical continuum fluids at constant pressure

We consider the application of finite-size scaling methods to isothermal-isobaric (constant-NpT) simulations of pure continuum fluids. A finite-size scaling ansatz is made for the dependence of the relevant scaling operators on the particle number. To test the proposed scaling form, constant pressure simulations of the Lennard-Jones fluid at its liquid-vapour critical point are performed. The critical scaling operator distributions are obtained and their scaling with particle number found to be consistent with the proposed behaviour. The forms of these scaling distributions are shown to be identical to their Ising model counterparts. The relative merits of employing the constant-NpT and grand canonical (constant-$\mu$VT) ensembles for simulations of fluid criticality are also discussed.