Importance of van der Waals interaction on structural, vibrational, and thermodynamics properties of NaCl

TL;DR

This study demonstrates that incorporating van der Waals interactions in DFT calculations significantly improves the accuracy of predicting structural, vibrational, and thermodynamic properties of NaCl, especially under extreme conditions.

Contribution

The paper introduces the use of van der Waals functionals in DFT to better model NaCl's properties, addressing limitations of traditional functionals.

Findings

Van der Waals functionals outperform standard DFT functionals in predicting NaCl properties.

Accurate phase boundary for B1 to B2 transition obtained.

Anharmonic effects are highly sensitive to the choice of exchange-correlation functional.

Abstract

Thermal equations of state (EoS) are essential in several scientific domains. However, experimental determination of EoS parameters may be limited at extreme conditions, therefore, {\it ab~initio} calculations have become an important method to obtain them. Density Functional Theory (DFT) and its extensions with various degrees of approximations for the exchange and correlation (XC) energy is the method of choice, but large errors in the EoS parameters are still common. The alkali halides have been problematic from the onset of this field and the quest for appropriate DFT functionals for such ionic and relatively weakly bonded systems has remained an active topic of research. Here we use DFT + van der Waals functionals to calculate vibrational properties, thermal EoS, thermodynamic properties, and the B1 to B2 phase boundary of NaCl. Our results reveal i) a remarkable improvement over the performance of standard Local Density Approximation and Generalized Gradient Approximation functionals for all these properties and phase transition boundary, as well as ii) great sensitivity of anharmonic effects on the choice of XC functional.