  Replica exchange molecular dynamics (REMD) becomes more efficient as the
frequency of swap between the temperatures is increased. Recently in [Plattner
et al, J. Chem. Phys. 135, 134111 (2011)] a method was proposed to implement
infinite swapping REMD in practice. Here we show that this method naturally
leads to a reformulation in terms of molecular dynamics simulations over a
mixture potential, which is both simple to implement in practice and provides a
better, energy based understanding of how to choose the temperatures in REMD to
optimize efficiency. It also opens the door to generalizations of REMD in which
the swaps involve other parameters than the temperature.
