A method to compute absolute free energies or enthalpies of fluids

We propose a new method to compute the free energy or enthalpy of fluids or disordered solids by computer simulation. The main idea is to construct a reference system by freezing one representative configuration, and then carry out a thermodynamic integration. We present a strategy and an algorithm which allows to sample the thermodynamic integration path even in the case of liquids, despite the fact that the particles can diffuse freely through the system. The method is described in detail and illustrated with applications to hard sphere fluids and solids with mobile defects.