Structural and Electronic Properties of Bulk $β$(2H)-GaSe from First-Principles DFT Calculations with van der Waals Corrections

Abstract

Gallium selenide (GaSe) is a layered III-VI semiconductor whose bulk crystal serves as the essential energetic reference for modeling the isolated monolayer. We present a systematic density-functional-theory (DFT) study of the centrosymmetric $β$(2H)-GaSe polymorph, employing the GGA-PBE functional supplemented with a semiempirical Grimme DFT-D2 dispersion correction to accurately capture the weak van der Waals interlayer coupling. The optimized in-plane lattice parameter agrees well with experimental values (within about 1.3%), while the out-of-plane parameter c is overestimated by the DFT-D2 correction relative to experiment, a known limitation of this dispersion scheme when the pairwise $C_6$ coefficients are not specifically fitted for the compound class under study. Electronic structure calculations confirm a non-magnetic, direct-gap profile (1.12 eV). Although this magnitude reflects the well-known underestimation of semilocal functionals, the qualitative band topology accurately captures the intrinsic two-dimensional carrier confinement within the bulk material. Consequently, this optimized three-dimensional framework provides a consistent energetic baseline for quantifying exfoliation processes, offering a reliable starting point for future theoretical explorations of the two-dimensional limit, such as surface functionalization for potential spintronic applications.