High-level calculations using internally contracted multireference configuration interaction including Davidson correction (icMRCI+Q) method have been carried out for the ground singlet states, the first excited states, and the lowest triplet states of a series of fluorine-substituted carbenes FCX (X = H, F, Cl, Br, and I). Equilibrium geometries and vibrational frequencies of the three electronic states, adiabatic transition energy of the first excited singlet state, as well as the ground singlet—lowest triplet energy gap (S-T gap) of each of FCX carbenes have been obtained. Effects of the basis set of icMRCI+Q calculation on the geometries and energies have been investigated. In addition, various corrections, including the scalar relativistic effect, spin-orbit coupling, and core-valence correlation, have been studied in calculating the transition energies and the S-T gaps, especially for heavy-atom carbenes. This results have been compared with previous calculations using a variety of methods. Our icMRCI+Q results are in very good agreement with the high-resolution laser-based spectroscopic results where available. Some structure and spectroscopic constants of the fluorine-substituted carbenes which are void in the literature have been provided with consistent high-level calculations. © 2013 Wiley Periodicals, Inc.