The Wannier functions (WF)^(35,36) are the most localized orbitals within the valence-band and conduction-band subspaces. Hence we will use WF as ϕₗ in Eqs (1) and (2). The WFs are mutually orthogonal, and the collective WFs for valence/conduction band fill in the valence/conduction band subspace. 2016 April 26, Jie Ma, Lin-Wang Wang, “Using Wannier functions to improve solid band gap predictions in density functional theory”, in Scientific Reports, volume 6, →DOI, article 24924, page 2:
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