High-Pressure Hydrogen Sulfide from First Principles: A Strongly Anharmonic Phonon-Mediated Superconductor

We use first-principles calculations to study structural, vibrational, and superconducting properties of H2S at pressures P >= 200 GPa. The inclusion of zero-point energy leads to two different possible dissociations of H2S, namely 3H(2)S -> 2H(3)S -> S and 5H(2)S -> 3H(3)S + HS2, where both H3S and HS2 are metallic. For H3S, we perform nonperturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electronphonon interaction (lambda approximate to 2.64 at 200 GPa) and T-c. Anharmonicity hardens H-S bond-stretching modes and softens H-S bond-bending modes. As a result, the electron-phonon coupling is suppressed by 30% (lambda approximate to 1.84 at 200 GPa). Moreover, while at the harmonic level T-c decreases with increasing pressure, the inclusion of anharmonicity leads to a T-c that is almost independent of pressure. High-pressure hydrogen sulfide is a strongly anharmonic superconductor.