Pyrite ${\mathrm{FeS}}_2$ has become the focus of many researchers in thin-film photovoltaics because it has some possibilities in photovoltaics. In this manuscript, we present an experimental and a theoretical study of the electronic structure of pyrite ${\mathrm{FeS}}_2$ alloyed with a small concentration of 1.19% of ruthenium (${\mathrm{Fe}}_{0.9881}{\mathrm{Ru}}_{0.0119}{\mathrm{S}}_2$) by using the Linear Muffin-Tin Orbital Method in the Atomic-Sphere approximation (LMTO-ASA) calculations and the density of states. We observed that the bandgap of ${\mathrm{FeS}}_2$ increases from $0.90508$ to $1.21586 \mathrm{eV}$ when we replace $~1.19\%$ of the $\mathrm{Fe}$ atoms with ruthenium atoms $\left(\mathrm{x}=0.0119 \mathrm{concentration} \mathrm{of} \mathrm{Ru}\right)$. We prove that this low concentration of Ru saved the gap states and the electronic and optical properties of ${\mathrm{FeS}}_2$ pyrite. Our calculated electronic bandgap is $1.21586 \mathrm{eV}$ and direct. Our results confirm that the symmetric operation of the space ${\mathrm{T}}_{\mathrm{h}}^6 \left(\mathrm{Pa}3\right)$ saves electronic structure of iron pyrite when alloyed with ruthenium.