Ab initio molecular electronic structure methods have been used to examine the rearrangement of methylnitrene (CH3N) to methyleneimine (CH2NH) on the lowest-lying singlet surface. Geometries have been optimized and harmonic vibrational frequencies obtained for both methylnitrene and methyleneimine as well as for the transition states connecting them. Basis sets up to triple zeta plus double polarization plus f functions (TZ2P+f) in size were used. Equilibrium geometries have been determined at the self-consistent field (SCF) and the single and double excitation configuration interaction (CISD) levels of theory. Because the 1E state of CH3N exhibits a Jahn-Teller splitting, the resulting 1A’ and 1A" states have been examined in order to determine their respective roles on the potential energy surface. After identifying and analyzing a larger 1A’-1A" splitting artifact due to symmetry breaking, the true classical Jahn-Teller splitting is predicted to be < 0.01 kcal mol-1. This theoretical investigation is the first to consider the effects of electron correlation in conjunction with extended basis sets on the energetics of the methylnitrene 1,2-hydrogen shift. It is found that the conclusion of most previous lower level studies-that there is no barrier to rearrangement on the 1A’ surface-appears to be justified. If C(s) symmetry is enforced, there is a sizable barrier (approximately 30 kcal mol-1 on the 1A" potential surface.