Finite-temperature infrared and Raman spectra of high-pressure hydrogen from first-principles molecular dynamics

Finite-temperature infrared and Raman spectra of theoretically proposed stable structures C 2 /c and P c of high-pressure solid hydrogen are calculated from time correlation functions of dipole moments and polarizabilities extracted from first-principles molecular dynamics simulations. Calculated spectra are much improved compared with those obtained from density functional perturbation theory at zero temperature, which suggests the significance of finite-temperature effects in both spectra. The excellent agreement between the calculated spectra of the C 2 /c structure and experimental results supports the theory that C 2 /c is the structure of phase III. The high-frequency Raman vibron mode of the P c structure is also well reproduced compared with experimental spectra. However, the energy of the low-frequency Raman vibron mode of the P c structure is underestimated up to 16%. This suggests that the atomic structure of the strongly bonded layer in phase IV is well predicted, while the weakly bonded layer still differs from the real structure somehow. In addition, we find that diffusion in the weakly bonded layer of the P c structure is strong and the layer displays several features of a two-dimensional liquid.