Electron-phonon coupling and exchange-correlation effects in superconducting H3S under high pressure
We investigate the H3S phase of sulfur hydride under high pressure similar or equal to 200 GPa by means of ab initio calculations within the framework of the density-functional theory with the PBE0 hybrid exchange-correlation (E-xc) approximation. The choice of E-xc has the largest effect on the calculated electron-phonon coupling (EPC) matrix elements; the high-pressure equation of state and phonon frequencies are only slightly modified. Mode-dependent EPC correction factors are determined from PBE0 using a frozen-phonon supercell approach, while standard density-functional perturbation theory is used to determine the EPC with PBE generalized-gradient approximation E-xc. Our principle finding is that the calculated PBE0 T-c is enhanced by 25% compared to PBE. This is similar in magnitude, but in opposite direction, to the proposed suppression of T-c by anharmonic effects [I. Errea et al., Phys. Rev. Lett. 114, 157004 (2015)]. Our calculations demonstrate the importance of considering exchange-correlation approximations for calculations of superconducting properties for this class of materials.