Software tools for thermodynamic calculation of mechanically unstable phases from first-principles data

This paper introduces the P4 software package, a set of software tools that automate the process of vibrational free energy calculation for mechanically unstable phases. The Piecewise Polynomial Potential Partitioning (P4) method is a recently developed method that tackles the issue of phonon instabilities in solid solutions and compounds. The method efficiently explores the system’s ab-initio energy surface by discrete sampling of local minima, which is combined with a continuous sampling of the vicinity of these local minima via a constrained harmonic lattice dynamic approach. The free energy values obtained by this toolkit can be used in thermodynamic assessments and phase diagram calculations. As a unique advantage, it provides the tool to calculate the vibrational free energy in a wide composition range in alloys. This feature is not practically achievable by relying only on brute-force molecular dynamics or random sampling techniques. Program Title: P4 (Piecewise Polynomial Potential Partitioning) Program Files doi: http://dx.doi.org/10.17632/sgfyhyrmjb.1 Licensing provisions: CC by 4.0 Programming language: C++ and Bash Nature of problem: A number of technologically relevant materials become thermodynamically stable at high enough temperatures in spite of existing phonon instabilities. This makes the standard lattice dynamics approaches insufficient to describe stabilizing entropy effects in these materials. Solution method: We implement the Piecewise Polynomial Potential Partitioning method [1] which facilitates thermodynamic calculations in mechanically unstable materials that are stabilized at finite temperatures due to anharmonic entropy effect. The current version of our model interfaces with density functional theory code VASP [2]. Additional comments including restrictions and unusual features: The P4 code needs to be built into the ATAT toolkit [3]. The instruction on how to install P4 into the ATAT toolkit is given in the documentation. Additional links for program files: http://go.uic.edu/p4, https://doi.org/10.26300/zs0n-2c84. References: [1] Sara Kadkhodaei, Qi-Jun Hong, and Axel van de Walle, Free energy calculation of mechanically unstable but dynamically stabilized bcc titanium. Phys. Rev. B, 95 (2017) 064101 [2] G. Kresse and J. Furthmuller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 54 (1996) 11169 [3] A. van de Walle and M. Asta and G. Ceder, The alloy theoretic automated toolkit: A user guide. Calphad 26 (2002) 539–553