Probing anharmonic phonons by quantum correlators: A path integral approach.

Area of Science:

• Computational Physics
• Materials Science
• Quantum Mechanics

Background:

• Accurate determination of vibrational properties is crucial for understanding material behavior.
• Path Integral Molecular Dynamics (PIMD) offers a quantum-mechanically accurate simulation method.
• Characterizing anharmonicity is essential for predicting thermodynamic and spectroscopic properties.

Purpose of the Study:

• To develop an efficient scheme for calculating vibrational properties from PIMD simulations.
• To accurately capture both temperature and quantum effects on anharmonicity.
• To provide reliable phonon spectra and anharmonicity strength estimations.

Main Methods:

• Utilizing zero-time Kubo-transformed correlation functions within PIMD.
• Developing two distinct estimators based on force-force and displacement-displacement correlators.
• Employing generalized eigenvalue equations for accelerated PIMD phonon calculations.
• Performing ab initio PIMD simulations for diamond and high-pressure atomic hydrogen.

Main Results:

• The developed estimators fully characterize phonon spectra and anharmonicity strength.
• The force-force correlator estimator provides accurate zero-point frequencies and thermodynamic properties in the weak anharmonic regime.
• The displacement-displacement correlator estimator accurately probes low-energy phonon excitations across varying anharmonicity.
• Ab initio PIMD simulations revealed stronger-than-expected anharmonicity in the I4₁/amd phase of atomic hydrogen.

Conclusions:

• The new PIMD-based scheme efficiently determines vibrational properties, including anharmonicity.
• Generalized eigenvalue equations significantly speed up PIMD phonon calculations.
• The study highlights substantial anharmonicity in high-pressure atomic hydrogen, impacting its vibrational spectrum.