Electronic States of CO Adsorbed onto Ni

I have calculated the electronic spectra of CO adsorbed onto Ni(001) in the $c(2\mathrm{}\times 2)$ structure. The $4\sigma$ band is found 10.8 eV below $E_{\mathrm{F}}$. The $1\pi$ and $5\sigma$ bands overlap to form one peak at - 7.9 eV below $E_{\mathrm{F}}$. This two-peak structure agrees with photoemission measurements. The results indicate that the chemisorbed CO molecular states are sufficiently extended so that dynamic final-state relaxation effects are negligible, and energy-level shifts are due to chemical bonding alone. Similar results are obtained for ${\mathrm{C}}_2$ ${\mathrm{H}}_4$ and ${\mathrm{C}}_2$ ${\mathrm{H}}_2$.