= \sum_{{\boldsymbol{\mathbf{k}}} \sigma \alpha'} \mathrm{e}^{i {\boldsymbol{\mathbf{k}}} \cdot {\boldsymbol{\mathbf{R}}}_j} \Phi^{\sigma \alpha'}_{\mu \alpha}({\boldsymbol{\mathbf{k}}}) c_{{\boldsymbol{\mathbf{k}}} \sigma \alpha'}