Excitons in solid molecular hydrogen from quantum Monte Carlo
arXiv preprint arXiv:2610.398942026
We use quantum Monte Carlo (QMC) methods to investigate the lowest neutral excitation of solid molecular hydrogen in phase-I under increasing compression, with emphasis on structural and response properties. We compare QMC wave functions with a single orbital substitution or a multideterminant expansion over every single particle-hole transition against the Bethe-Salpeter equation (BSE) and time-dependent density-functional theory (TDDFT). The multideterminant expansion barely changes QMC excitation energies but is crucial for exciton localization, transition dipole moments, and excitation-induced forces. These differences decrease upon compression, as the system approaches a semiconducting regime where multiple particle-hole transitions are less important.
