  Theoretical material investigation based on density functional theory (DFT)
has been a breakthrough in the last century. Nevertheless, the optical
properties calculated by DFT generally show poor agreement with experimental
results particularly when the absorption-coefficient ({\alpha}) spectra in
logarithmic scale are compared. In this study, we have established an
alternative DFT approach (PHS method) that calculates highly accurate {\alpha}
spectra, which show remarkable agreement with experimental spectra even in
logarithmic scale. In the developed method, the optical function estimated from
generalized gradient approximation (GGA) using very high-density k mesh is
blue-shifted by incorporating the energy-scale correction by a hybrid
functional and the amplitude correction by sum rule. Our simple approach
enables high-precision prediction of the experimental {\alpha} spectra of all
solar-cell materials (GaAs, InP, CdTe, CuInSe2 and Cu2ZnGeSe4) investigated
here. The developed method is superior to conventional GGA, hybrid functional
and GW methods and has clear advantages in accuracy and computational cost.
