Free energy evaluation in field-theoretic polymer simulations.

We present a thermodynamic integration method for free energy evaluation in field-theoretic simulations of classical fluids and polymers. The approach employs an Einstein crystal reference state, analogous to a method developed for particle simulations of crystals by Frenkel and Ladd, but applies equally well in the present context to ordered and disordered phases. Thermodynamic averages are computed using complex Langevin sampling, which is effective against the sign problem inherent to polymer field theories. Our method is illustrated in the context of a diblock copolymer melt, where we provide a demonstration of the experimentally observed transition between the cubic gyroid and disordered phases.