Frozen-phonon method for state anticrossing situations and its application to zero-point motion effects in diamondoids

The frozen-phonon method, used to calculate electron-phonon coupling effects, requires calculations of the investigated structure using atomic coordinates displaced according to certain phonon eigenmodes. The process of "freezing in"the specific phonon can bring electronic eigenstates that are energetically close in energy into an anticrossing. This electronic anticrossing effect is, however, unrelated to electron-phonon coupling and needs to be removed if the eigenvalues are used to obtain electron-phonon coupling. We present a procedure for how to deal with these problematic anticrossing situations and apply it to the band gap zero-point motion renormalization of 16 diamondoids and urotropine using different exchange correlation functionals. From the 17 structures, 5 require the correction presented here, leading to a modification of the zero-point renormalization between 4% and 185%. We find gap renormalizations in the range of -150 to -370meV for diamondoids but only around -40meV for urotropine.