Ab Initio Water Pair Potential with Flexible Monomers

A potential energy
surface for the water dimer with explicit dependence
on monomer coordinates is presented. The surface was fitted to a set
of previously published interaction energies computed on a grid of
over a quarter million points in the 12-dimensional configurational
space using symmetry-adapted perturbation theory and coupled-cluster
methods. The present fit removes small errors in published fits, and
its accuracy is critically evaluated. The minimum and saddle-point
structures of the potential surface were found to be very close to
predictions from direct ab initio optimizations. The computed second
virial coefficients agreed well with experimental values. At low temperatures,
the effects of monomer flexibility in the virial coefficients were
found to be much smaller than the quantum effects.