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## Simulation of planar systems at constant normal pressure : Is it also possible to keep the plate separation constant?

Constant pressure simulations of planar systems (at fixed particle number) usually require the fluctuations of the conjugate length: When, for example, the temperature is increased, the film thickness D increases in order to keep the normal pressure P-N constant. Obviously, P-N can also be kept constant by leaving D unchanged but varying the surface area A. However, as P-N and A are not conjugate variables, a proper realization of this idea is not straightforward. Using the fact that A is conjugate to the lateral pressure P-T and that P-T is related to P-N via the surface free energy, we present an iterative method which searches for that value of P-T corresponding exactly to the desired normal pressure. It is shown that a simulation at this value of P-T ensures that the time average of the normal pressure is indeed equal to the desired value. Analytic arguments are given for the convergence of the method in its initial formulation. Furthermore, an empirical rule is presented which improves the rate of convergence by an order of magnitude in simulation time. The present method allows to separate the effects of the temperature from that of the film thickness, and thus, is particularly useful for the study of ultra thin films where a variation of the film thickness can have crucial effects on system properties.