Harmonic-to-anharmonic thermodynamic integration made simple using REG TI

Area of Science:

• Computational chemistry
• Solid-state physics
• Thermodynamics

Background:

• Standard thermodynamic integration (TI) faces numerical challenges with solids having diffusive degrees of freedom.
• This singularity hinders accurate free energy estimation in systems like solids with rotating groups or migrating defects.

Purpose of the Study:

• Introduce a novel regularization method to address the singularity in TI.
• Improve the accuracy and efficiency of anharmonic free energy calculations for solids.

Main Methods:

• Developed Regularized Endpoint Gradient (REG) TI, a simple regularization technique.
• Applied REG TI to a model system and paracetamol polymorphs.
• Evaluated the method's ability to handle near-singular integrands.

Main Results:

• REG TI successfully removes the near singularity in the TI integrand.
• The method yields a well-behaved integrand suitable for uniform grid evaluation.
• Demonstrated accurate prediction of relative stability for paracetamol polymorphs.

Conclusions:

• REG TI offers a robust solution for anharmonic free energy calculations in solids.
• The method simplifies and potentially automates these complex calculations.
• REG TI is expected to be broadly applicable to solids with diffusive degrees of freedom.