Freezing by Monte Carlo phase switch

We describe a Monte Carlo procedure which allows sampling of the disjoint c onfiguration spaces associated with crystalline and fluid phases, within a single simulation. The method utilizes biased sampling techniques to enhanc e the probabilities of gateway states tin each phase which are such that a global switch (to the other phase) can be implemented. Equilibrium freezing -point parameters can be determined directly, statistical uncertainties pre scribed transparently, and finite-size effects quantified systematically. T he method is potentially quite general. We apply it to the freezing of hard spheres.