Density functional theory First-principles Optical properties -Ga2O3 Electronic structure Electronic structure and optical properties Energy gap Generalized gradient approximations HCl Hubbard U TiO2 density functional theory first-principles 1ST PRINCIPLES 3.5-PERCENT NACL ALUMINUM ANATASE Absorption spectroscopy BROOKITE Band structure CARBON-STEEL CORROSION Ca-doped &#945 Ca-doped α-Ga2O3 Calcium Calculated optical properties Calculations Carbon Charge density distributions Chemical bonds Complex dielectric functions