  A transferable potential energy function for describing the interaction
between water molecules is presented. The electrostatic interaction is
described rigorously using a multipole expansion. Only one expansion center is
used per molecule to avoid the introduction of monopoles. This single center
approach turns out to converge and give close agreement with ab initio
calculations when carried out up to and including the hexadecapole. Both dipole
and quadrupole polarizability is included. All parameters in the electrostatic
interaction as well as the dispersion interaction are taken from ab initio
calculations or experimental measurements of a single water molecule. The
repulsive part of the interaction is parametrized to fit ab initio calculations
of small water clusters and experimental measurements of ice Ih. The
parametrized potential function was then used to simulate liquid water and the
results agree well with experiment, even better than simulations using some of
the point charge potentials fitted to liquid water. The evaluation of the new
interaction potential for condensed phases is fast because point charges are
not present and the interaction can, to a good approximation, be truncated at a
finite range.
