Hydrogen phase-IV characterization by full account of quantum anharmonicity

We devise a framework to compute accurate phonons in molecular crystals even in case of strong quantum anharmonicity. Our approach is based on the calculation of the static limit of the phononic Matsubara Green's function from path integral molecular dynamics simulations. Our method enjoys a remarkably low variance, which allows one to compute accurate phonon frequencies after a few picoseconds of nuclear dynamics, and it is further stabilized by the use of appropriate constrained displacement operators. We applied it to solid hydrogen at high pressure. For phase III, our predicted infrared (IR) and Raman active vibrons agree very well with experiments. We then characterize the crystalline symmetry of phase IV by direct comparison with vibrational data and we determine the character and isotopic shift of its Raman and IR vibron peaks.

Automated summary

A new method for computing phonons in molecular crystals under strong quantum anharmonicity is proposed and successfully applied to solid hydrogen at high pressure. The method, based on path integral molecular dynamics simulations, shows remarkably low variance and accuracy in predicting phonon frequencies. Experimental validation of the results for different phases is also provided.