As a first step toward the construction of a single-valued double many-body expansion potential energy surface for CHNO((3)A), we have carried out CASSCF and CASPT2 calculations of six diatomic curves, involving a total of nine electronic states. The nb initio curves have been represented analytically using the extended Hartree-Fock approximate correlation energy model. In all cases, the semiempirical curves have been found to agree well with the available spectroscopic RKR data.