  The theoretical description of the thermodynamics of water is challenged by
the structural transition towards tetrahedral symmetry at ambient conditions.
As perturbation theories typically assume a spherically symmetric reference
fluid, they are incapable of accurately describing the liquid properties of
water at ambient conditions. In this paper we solve this problem, by
introducing the concept of an associated reference perturbation theory (APT).
In APT we treat the reference fluid as an associating hard sphere fluid which
transitions to tetrahedral symmetry in the fully hydrogen bonded limit. We
calculate this transition in a theoretically self-consistent manner without
appealing to molecular simulations. This associated reference provides the
reference fluid for a second order Barker-Hendersen perturbative treatment of
the long-range attractions. We demonstrate that this new approach gives a
significantly improved description of water as compared to standard
perturbation theories.
