  First principles calculations of the geometric and electronic structures have
been performed for two coverages (0.25 ML and 0.5 ML) of C on Ni(001) to
understand the mechanism of the Ni(001) reconstruction induced by carbon
adsorption. The calculated structural behavior of the system is in a good
agreement with experimental observations. The calculated path and energetics of
the $c(2\times 2)$ -- $p4g$ reconstruction in C$_{0.5}$/Ni(001) is provided. A
dramatic reduction of the local electronic charge on adsorbed carbon is found
to occur upon the reconstruction that decreases the electron-electron repulsion
on C site. This effect together with the formation of covalent bonds between C
and the second layer Ni atoms, leads to reconstruction of Ni(001).
