The adsorption of formyl radicals (HCO) on Ni(100) is treated using a many-electron embedding theory, modeling the lattice as a 30-atom, three-layer cluster with the Ni atoms fixed at bulk. Ab initio valence orbital configuration interaction (multiple parent) calculations carried out on a local surface region permit an accurate description of bonding at the surface. The 3d orbitals are explicitly for six Ni atoms in the local surface region. The Ni(100) potential surface is very flat for eta(1)-formly-C adsorption. eta(1)-Formyl-C binds to the surface mainly via the C atom, and the energy minimum occurs for an O-C-surface normal angle of 90 degrees, 90 degrees, and 110 degrees at four-fold, bridge, and atop sites, respectively. The HCO bond angle is 120 degrees. Calculated adsorption energies are 63.7, 63.5 and 63.6 kcal/mol at four-fold, bridge, and atop sites, with C-surface distances of 1.93, 1.94, and 2.04 Angstrom respectively. Calculated C-O stretching frequencies are around 1760 cm(-1) and C-H stretching frequencies are around 2940 cm(-1) for HCO at all adsorption sites and equilibrium geometries. The bonding of formyl to the nickel surface involves ionic and covalent contributions and substantial mixing with Ni 3d orbitals. Calculated eta(1)-formyl-O, bonding to the surface via the O atom, is energetically less stable than eta(1)-formyl-C by 16.3 kcal/mol. Calculated C-O and C-H stretching frequencies are 1370 and 2988 cm(-1) for eta(1)-formyl-O at four-fold sites. No energy barrier occurs for the conversion of eta(1)-formyl-O to eta(1)-formyl-C.