Excited state forces code. Calculate forces after excitation by combining results from GW/BSE and DFPT calculations.
The excited force expression is given by:
[ F = \sum_{\nu cvc'v' k} A_{cvk} A^*{c'v'k} (g^{\nu}{cc'k} - g^{\nu}_{vv'k}) \hat{\nu} ]
where (\hat{\nu}) is one displacement pattern (a phonon mode for example), (A_{cvk}) is the exciton coefficient obtained from the Bethe-Salpeter Equation, and (g^{\nu}{ijk}) is the electron-phonon coefficient (\langle ik | \partial{\nu} V | ij \rangle).