Rationalization of the use of harmonic electron-phonon coupling matrix

[ZhongyuWAN]

Dear SSCHA developers,

I have a question regarding the fundamental principles of SSCHA and hope to receive your assistance. In Tutorial 7, "Calculation of the electron-phonon interaction and superconducting properties with the SSCHA," I noticed a step where the SSCHA dynamical matrix is preceded by the harmonic approximation dynamical matrix calculated using the QE package. This raised a question for me, considering the example of PdH, which is a strongly anharmonic crystal. Since the electron-phonon coupling matrix elements require both the dynamical matrix and the derivatives of the KS potential, while SSCHA can generate anharmonic dynamical matrices, can SSCHA also produce the derivatives of the KS potential? Quantum anharmonic effects can alter lattice and atomic displacements, so the derivatives of the anharmonic KS potential must differ from those under the harmonic approximation.

If the derivatives of the anharmonic KS potential can be computed by SSCHA, where are they stored? From the tutorial, I observed that a series of processes revolve around the harmonic electron-phonon coupling matrix elements. Is it reasonable to do so for a system with strong anharmonicity?

With regards,
Zhongyu

[DjordjeDangic]

Hello Zhongyu,

Please read the following paper in order to better understand electron-phonon coupling https://arxiv.org/abs/2303.02621 in these types of materials. This approach is not implemented in SSCHA.

Currently, we are treating electron-phonon as it is laid out in the tutorial you are mentioning. Is it reasonable? It is not less reasonable than purely harmonic calculation.