  We present an ab initio calculation to understand electronic structures and
optical properties of a tungsten carbide WC being a major component of a
TiCN-based cermet. We found that the WC has a fairly low-energy plasma
excitation $\sim$0.6 eV (2 $\mu$m) and therefore can be a good constituent of a
solar selective absorber. The evaluated figure of merit for photothermal
conversion is prominently high compared to those of the other materials
included in the TiCN-based cermet. The imaginary part of the dielectric
function is considerably small around the zero point of the real part of the
dielectric function, corresponding to the plasma excitation energy. Therefore,
a clear plasma edge appeared, ensuring the high performance of the WC as the
solar absorber.
