  We present a new type of the Hamiltonian replica-exchange method, in which
not temperatures but the van der Waals radius parameter is exchanged. By
decreasing the van der Waals radii that control spatial sizes of atoms, this
Hamiltonian replica-exchange method overcomes the steric restrictions and
energy barriers. Furthermore, the simulation based on this method escapes from
the local-minimum free-energy states and realizes effective sampling in the
conformational space. We applied this method to an alanine dipeptide in aqueous
solution and showed the effectiveness of the method by comparing the results
with those obtained from the conventional canonical method.
