Ab initio molecular electronic structure theory has been used to determine the energy separation between the lowest {sup 3}B{sub 1} and {sup 1}A states of dimethylcarbene. The geometries of both states have been optimized at the self-consistent Field(SCF) and the single and double excitation configurationinteraction (CISD) levels of theory using various basis sets. Single point energies at the coupled cluster with single and double excitations (CCSD) and the CCSD with perturbative triple excitations [CCSD(T)] levels of theory were determined at the CISD equilibrium geometries with the same basis set. Harmonic vibrational frequencies and infrared (IR) intensities were determined for both states at the SCF level of theory using all three basis sets and at the CISD level of theory - using the DZP and TZ2P basis sets. The energy separation between the lowest triplet state ({sup 3}B{sub 1}) and the lowest singlet state ({sup 1}A) for dimethylcarbene decreases with increasing basis set size and electron correlation. At the highest level of theory employed in this research, TZ2P+fCCSD(T), the singlet state is predicted to be lower in energy than the triplet state by 0.8 kcalmol{sup -1}. This energy separation becomes 1.4 kcal mol{sup -1} with the inclusion of zero-point vibrational energy(ZPVE)more » corrections. 35 refs., 2 figs., 4 tabs.« less