Quantum Nuclear Dynamics: Multiphonon scattering and Non-Gaussian Fluctuations

Hydrogen is the simplest element in the periodic table. Under ambient conditions, it exists as a molecular gas, but with increasing pressure, it forms a molecular solid at around 5 GPa. In this presentation, I will discuss the phase diagram of solid hydrogen at 0 K and high pressure. Experimentally, only Raman scattering and infrared (IR) absorption provide information about the underlying structure. From a theoretical perspective, there is still debate regarding the crystal configuration, as hydrogen exhibits complex electronic correlations and strong quantum anharmonic fluctuations.

I will demonstrate that IR and Raman spectra simulations can help distinguish among the proposed structures. To do so, I will introduce the Time-Dependent Self-Consistent Harmonic Approximation (TDSCHA), which approximates the atomic density matrix as the most general Gaussian. This method provides nuclear time-dependent responses, including quantum and anharmonic effects.

In the final part of the presentation, I will show that approximating the quantum nuclear fluctuations of nuclei as Gaussian misses rotational modes and tunnelling effects. However, I will demonstrate that we can generalise the method by employing a nonlinear invertible transformation.

If you would like to meet with Antonio during his visit, please contact Michael Fechner.