An optimized molecular model for ammonia

TL;DR

This paper presents an optimized molecular model for ammonia that integrates quantum mechanical data and experimental vapor-liquid equilibrium data, achieving high accuracy in predicting thermophysical properties across various phases.

Contribution

The study introduces a new ammonia model that combines quantum calculations and experimental data, improving predictive accuracy without using structural information in optimization.

Findings

Mean unsigned deviations: 0.7% in saturated liquid density

1.6% in vapor pressure, 2.7% in vaporization enthalpy

Accurately predicts thermophysical properties and radial distribution functions

Abstract

An optimized molecular model for ammonia, which is based on a previous work of Kristoef et al., Mol. Phys. 97 (1999) 1129--1137, is presented. Improvements are achieved by including data on geometry and electrostatics from quantum mechanical calculations in a first model. Afterwards the parameters of the Lennard-Jones potential, modeling dispersive and repulsive interactions, are optimized to experimental vapor-liquid equilibrium data of pure ammonia. The resulting molecular model shows mean unsigned deviations to experiment of 0.7% in saturated liquid density, 1.6% in vapor pressure, and 2.7% in enthalpy of vaporization over the whole temperature range from triple point to critical point. This new molecular model is used to predict thermophysical properties in the liquid, vapor and supercritical region, which are in excellent agreement with a high precision equation of state, that was optimized to 1147 experimental data sets. Furthermore, it is also capable to predict the radial distribution functions properly, while no structural information is used in the optimization procedure.