Gibbs free-energy differences between polymorphs via a diabat approach

Area of Science:

• Materials Science
• Computational Chemistry
• Thermodynamics

Background:

• Polymorph free-energy differences are crucial for predicting material properties and behavior.
• Accurate calculation of these differences is essential for materials design and discovery.
• Existing methods may have limitations in certain thermodynamic ensembles or with specific potential energy surface characteristics.

Purpose of the Study:

• To extend the diabat interpolation framework for calculating Gibbs free-energy differences between polymorphs.
• To adapt the Zwanzig-Bennett relation for the NPT ensemble, enabling direct Gibbs free-energy calculations.
• To demonstrate the applicability of the method for non-parabolic diabats and compare it with established techniques.

Main Methods:

• Extension of the Zwanzig-Bennett relation to the isobaric-isothermal (NPT) ensemble.
• Application of a diabat interpolation framework utilizing quadratic interpolation of diabats.
• Illustrative calculations for Gibbs free-energy differences between BCC and HCP phases of zirconium.
• Comparison with the conventional lattice switch Monte Carlo method.

Main Results:

• The extended Zwanzig-Bennett relation directly yields Gibbs free-energy differences in the NPT ensemble.
• The diabat method provides accurate free-energy estimates even when diabats are not parabolic.
• Successful application to calculate the Gibbs free-energy difference for zirconium polymorphs (BCC and HCP).
• The diabat method shows comparable or improved performance against the lattice switch Monte Carlo approach.

Conclusions:

• The diabat interpolation framework, extended to the NPT ensemble, offers a robust method for calculating polymorph free-energy differences.
• This approach simplifies free-energy calculations and expands the applicability of diabat methods.
• The study validates the method for practical materials science applications, exemplified by zirconium.