Dispersion interactions in proposed covalent superhydride superconductors

Recent developments in high temperature superconducting materials under high pressure have made numerical evaluation of the superconducting transition temperature (Tc) of predicted materials critically important as a means to identification. Existing methods of calculating Tc often do not agree with each other or experiment, often due to the large number of complex factors that contribute to this property; among them is the neglect of dispersion interactions in commonly used density functionals. Here, we evaluate the effect of including dispersion interactions on the predicted superconducting properties of two examples of the covalent superhydride class of very high-Tc superconducting materials. In both cases, dispersion is found to have sizable effects, increasing the electron-phonon coupling as compared to the reference case of elemental niobium. A detailed investigation traces the origin of this effect in a 270 GPa R3m CSH7 [Cui et al., Phys. Rev. B 101, 134504 (2020)] structure to structural distortions driven by long-range electron-phonon interactions rather than novel bonding networks.