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G (P,T)

GPT is a set of scripts that automate the Gibbs free energy calculation using Vienna ab-initio Simulation Package (VASP). While GPT is designed for VASP, it is flexible enough that other first principles method can also be used.

There are basically two approaches to calculate Gibbs free energy. One approach uses molecular dynamics or Monte Carlo methods to sample the configuration space and then calculate the Gibbs free energy from ensemble average. The other approach uses quasi-harmonic approximation to estimate the free energy contribution due to ion motion. While quasi-harmonic approximation is limited to temperature below melting, it is generally much less computationally demanding especially for first principles methods.

GPT calculates the Gibbs free energy using quasi-harmonic approximation. Features of the GPT scripts include:

• Phonon calculation using PHON program with my enhancements;
• Calculation of Born effective charge;
• Optical and static dielectric constant;
• LO/TO splitting
• Elastic constant, bulk modulus
• Helmholtz free energy F(V,T)
• Gibbs free energy G(P,T)

You can find user manual and examples here.

***This package is still under test.***

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