High pressure ionic and molecular crystals of ammonia monohydrate within density functional theory

TL;DR

This study uses density functional theory to predict high-pressure phases of ammonia monohydrate, revealing a proton-transfer phase stabilized at higher pressures when more accurate functionals are used, aligning with experimental observations.

Contribution

It introduces a new predicted proton-transfer phase of ammonia monohydrate and evaluates the accuracy of different density functionals in modeling high-pressure behavior.

Findings

PBE over-stabilizes proton transfer phases due to charge over-movement.

Hybrid PBE0 functional corrects over-binding, raising transition pressure.

Proton transfer phase predicted to occur above 10 GPa, consistent with experiments.

Abstract

A combination of first-principles density functional theory calculations and a search over structures predicts the stability of a proton-transfer modification of ammonia monohydrate with space group P4/nmm. The phase diagram is calculated with the PBE density functional, and the effects of a semi-empirical dispersion correction, zero point motion, and finite temperature are investigated. Comparison with MP2 and coupled cluster calculations shows that the PBE functional over-stabilizes proton transfer phases because too much electronic charge moves with the proton. This over-binding is partially corrected by using the PBE0 hybrid exchange-correlation functional, which increases the enthalpy of P4/nmm by about 0.6 eV per formula unit relative to phase I of ammonia monohydrate (AMH-I) and shifts the transition to the proton transfer phase from the PBE pressure of 2.8 GPa to about 10 GPa. This is consistent with experiment as proton transfer phases have not been observed at pressures up to ~9 GPa, while higher pressures have not yet been explored experimentally.