# phq: a Fortran code to compute phonon quasiparticle properties and dispersions

## TL;DR

This paper introduces a Fortran code that calculates anharmonic phonon properties and free energy of solids from first-principles, using a phonon quasiparticle approach based on molecular dynamics simulations.

## Contribution

The code uniquely combines phonon quasiparticle extraction from MD with harmonic force constant construction to compute anharmonic phonon dispersions and free energy.

## Key findings

- Successfully applied to silicon in diamond structure
- Accurately computes temperature-dependent anharmonic phonons
- Provides a practical tool for first-principles anharmonic calculations

## Abstract

We here introduce a Fortran code that computes anharmonic free energy of solids from first-principles based on our phonon quasiparticle approach. In this code, phonon quasiparticle properties, i.e., renormalized phonon frequencies and lifetimes, are extracted from mode-projected velocity auto-correlation functions (VAF) of modes sampled by molecular dynamics (MD) trajectories. Using renormalized frequencies as input, the code next constructs an effective harmonic force constant matrix to calculate anharmonic phonon dispersions over the whole Brillouin zone and thus the anharmonic free energy in the thermodynamic limit ($N \rightarrow \infty$). We illustrate the use of this code to compute ab initio temperature-dependent anharmonic phonons of Si in the diamond structure.