Ab initio molecular electronic structure theory has been used to determine the energy separation between the lowest B-3(1) and (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 configuration interaction (CISD) levels of theory using the double-zeta plus polarization (DZP), the triple-zeta plus double polarization (TZ2P), and the TZ2P basis set with a set of higher angular momentum functions on the central carbon atom (TZ2P+f) basis sets. For singlet dimethylcarbene, structures have also been optimized using the two reference CISD method. 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 (B-3(1)) and the lowest singlet state ((1)A) for dimethylcarbene decreases with increasing basis set size and electron correlation. At the highest level of theory employed in this research, TZ2P+f CCSD(T), the singlet state is predicted to be lower in energy than the triplet state by 0.8 kcal mol(-1). This energy separation becomes 1.4 kcal mol(-1) with the inclusion of zero-point vibrational energy (ZPVE) corrections.