  The high-energy tail of the distribution of solute-solvent interaction
energies is poorly characterized for condensed systems, but this tail region is
of principal interest in determining the excess free energy of the solute. We
introduce external fields centered on the solute to modulate the short-range
repulsive interaction between the solute and solvent. This regularizes the
binding energy distribution and makes it easy to calculate the free energy of
the solute with the field. Together with the work done to apply the field in
the presence and absence of the solute, we calculate the excess chemical
potential of the solute. We present the formal development of this idea and
apply it to study liquid water.
