# A perturbation theory for water with an associating reference fluid

## TL;DR

This paper introduces an associated reference perturbation theory (APT) for water, modeling its transition from spherical to tetrahedral symmetry, resulting in more accurate thermodynamic predictions than standard methods.

## Contribution

The paper develops a novel APT approach that accounts for water's structural transition, improving upon traditional perturbation theories by self-consistently modeling hydrogen bonding effects.

## Key findings

- Significantly improved thermodynamic description of water.
- Self-consistent modeling of tetrahedral transition.
- Enhanced accuracy over standard perturbation theories.

## Abstract

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.

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Source: https://tomesphere.com/paper/1706.05725