Vibrational properties of solid molecular hydrogen through quantum correlators

In the complex phase diagram of solid hydrogen at high pressures, phase transitions are indirectly deduced from the analysis of infrared and Raman spectra by comparison of experimental data with theoretical predictions. However, anharmonic and nuclear quantum effects (NQE) are notoriously large in hydrogen and hydrogen-based materials, and pose a formidable challenge for theory predictions. In this work we propose a generalization of the method presented in Ref. [1], in order to accurately describe anharmonicity and NQE in molecular crystals where the interplay between molecular and lattice degrees of freedom plays a pivotal role to determine their vibrational properties. We show that our approach is able to clearly distinguish between different competing candidates for hydrogen phase III, thanks to an accurate determination of vibronic frequencies. Our scheme therefore represents a valuable tool to identify high-pressure molecular structures from first principles through their vibronic characterization. 

[1] - TM et al., “Probing anharmonic phonons by quantum correlators: A path integral approach”, J. Chem. Phys. 154, 224108 (2021)