  We report a permutationally invariant polynomial (PIP) potential energy
surface for the water 4-body interaction. This 12-atom PES is a fit to 2119,
symmetry-unique, CCSD(T)-F12a/haTZ (aug-cc-pVTZ basis for 'O' atom and cc-pVTZ
basis for 'H' atom) 4-b interaction energies. These come from low-level,
direct-dynamics calculations, tetramer fragments from an MD water simulation at
300 K, and from the water hexamer, heptamer, decamer, and 13-mer clusters. The
PIP basis is purified to ensure that the 4-b potential goes rigorously to zero
in monomer+trimer and dimer+dimer dissociations for all possible such
fragments. The 4-b energies of isomers of the hexamer calculated with the new
surface are shown to be in better agreement with benchmark CCSD(T) results than
those from the MB-pol potential. Other tests validate the high-fidelity of the
PES.
