Franck-Condon Factors are calculated for the C2− and CO molecules and are used to identify these species in the optical spectra of comet C/2000WM1(LINEAR) and C/2000 C1 (Ikeya–Zhang). The present results compare well with those obtained by other methods. The optical spectra were recorded by means of the 2.12-m telescope at the Guillermo Haro observatory in Cananea, Sonora, México. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem 107: 2650–2653, 2007