-Ga2O3 Density functional theory Electronic structure Electronic structure and optical properties Energy gap First-principles Generalized gradient approximations density functional theory first-principles Absorption spectroscopy Band structure Ca-doped &#945 Ca-doped α-Ga2O3 Calcium Calculated optical properties Calculations Charge density distributions Chemical bonds Complex dielectric functions Crystal structure Density of states Electrical and optical properties Exchange-correlation potential First-principles calculation First-principles study GA2O3 GROWTH Gallium compounds High-power transistors Inter-band transition