DFT-D3 density functional computations of molecular structure and bond interactions of quasi-2D Cu ₂ WSe ₄ ternary chalcogenide slab for optoelectronic and photovoltaic applications

Abstract A comprehensive first-principles investigation of the structural, electronic, and optical properties of the 2D Cu 2 WSe 4 slab is presented using three-dimensional density functional theory calculations as implemented in the SIESTA code. The optimized crystal structure is analysed in terms of its electronic band structure and orbital-resolved density of states, revealing an indirect band gap with the valence-band maximum along the M –Γ direction and the conduction-band minimum located at the Γ point. Detailed crystal orbital overlap population/crystal orbital Hamiltonian population analysis is employed to elucidate the nature of Cu–Se and W–Se bonding interactions and their influence on the formation of valence and conduction states. Optical properties are examined within the independent-particle approximation, where the absence of low-energy absorption is shown to originate from the indirect nature of the band gap, while prominent absorption features at higher photon energies arise from direct interband transitions involving Se- p and W- d states. By correlating electronic structure, chemical bonding, and optical response, this work provides a unified microscopic understanding of the optical behaviour of Cu 2 WSe 4 and highlights the importance of bonding-driven electronic transitions beyond simple band-gap considerations. The present results offer valuable insight for the rational design and optimization of Cu-based chalcogenide materials for optoelectronic and photovoltaic applications.