The orthorhombic $\mathrm{LiGa}{\mathrm{S}}_2$ is widely used in nonlinear optical devices due to its wide direct optical band gap (4.15 eV). Nonetheless, its large hole effective mass results in low hole mobility, which severely limits its applications as p-type semiconductor. In this work, the tailored alloying strategy and valence band engineering scheme are adopted to raise the valence band maximum and decrease the hole effective mass. Based on the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional calculations, the structural, electronic, and optical properties and stability for $\mathrm{L}{\mathrm{i}}_{1-x}\mathrm{C}{\mathrm{u}}_x\mathrm{Ga}{\mathrm{S}}_2$ alloys are predicted. The unfolded band structure reveals that Cu alloying primarily modifies the valence bands. Higher Cu-$3d$ orbitals can elevate the valence band maximum of the alloys. The enhanced $p\text{−}d$ hybridization in the valence bands significantly reduces the hole effective mass. Specifically, along the Γ-Y direction, the hole effective mass for $\mathrm{L}{\mathrm{i}}_{0.625}\mathrm{C}{\mathrm{u}}_{0.375}\mathrm{Ga}{\mathrm{S}}_2$ alloy is reduced from $1.95m_0$ to $0.494m_0$, where $m_0$ is the free electron mass, representing a 75% reduction. Moreover, the excellent visible light transparency for $\mathrm{L}{\mathrm{i}}_{0.625}\mathrm{C}{\mathrm{u}}_{0.375}\mathrm{Ga}{\mathrm{S}}_2$ alloy is preserved. These findings provide an important insight into the design of optoelectronic devices based on $\mathrm{L}{\mathrm{i}}_{0.625}\mathrm{C}{\mathrm{u}}_{0.375}\mathrm{Ga}{\mathrm{S}}_2$ alloy.

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• Electronic structure
• First-principles calculations
• Alloys
• Semiconductor compounds
• Semiconductors
• Wide band gap systems
• Hybrid functionals