  This article describes nonequilibrium techniques for the calculation of free
energies of solids using molecular dynamics (MD) simulations. These methods
provide an alternative to standard equilibrium thermodynamic integration
methods and often present superior efficiency. Here we describe the
implementation in the LAMMPS (Large-scale Atomic/Molecular Massively Parallel
Simulator) code of two specific nonequilibrium processes that allow the
calculation of the free-energy difference between two different system
Hamiltonians as well as the free-energy temperature dependence of a given
Hamiltonian, respectively. The theory behind the methods is summarized, and we
describe (including fragments of LAMMPS scripts) how the process parameters
should be selected to obtain the best-possible efficiency in the calculations
of free energies using nonequilibrium MD simulations. As an example of the
application of the methods we present results related to polymorphic
transitions for a classical potential model of iron.
