spGroup

spGroup is a web application for determining electron-phonon selection rules from crystal symmetry. Given a space group and the wavevectors of an initial electron state, a phonon, and a final electron state, it checks momentum conservation, computes the small group of the interaction, and reports the number of independent reduced matrix elements $a_1$ allowed by symmetry. An $a_1$ of zero means the interaction is symmetry-forbidden; a nonzero value indicates it is symmetry-allowed, though the actual interaction strength still requires first-principles methods such as DFPT/EPW.