  Spherical truncations of Coulomb interactions in standard models for water
permit efficient molecular simulations and can give remarkably accurate results
for the structure of the uniform liquid. However truncations are known to
produce significant errors in nonuniform systems, particularly for
electrostatic properties. Local molecular field (LMF) theory corrects such
truncations by use of an effective or restructured electrostatic potential that
accounts for effects of the remaining long-ranged interactions through a
density-weighted mean field average and satisfies a modified Poisson's equation
defined with a Gaussian-smoothed charge density. We apply LMF theory to three
simple molecular systems that exhibit different aspects of the failure of a
naive application of spherical truncations -- water confined between
hydrophobic walls, water confined between atomically-corrugated hydrophilic
walls, and water confined between hydrophobic walls with an applied electric
field. Spherical truncations of 1/r fail spectacularly for the final system in
particular, and LMF theory corrects the failings for all three. Further, LMF
theory provides a more intuitive way to understand the balance between local
hydrogen bonding and longer-ranged electrostatics in molecular simulations
involving water.
