Nine conformers were identified for N,N,N',N'-tetramethylethylenediamine (TMEDA) with a number of theoretical methods, and their relative energies were compared. The heats of formation at 298 K (Delta H degrees(f298)) were determined at the G3(MP2) level. The Delta H degrees(f298) value for the most stable conformer is -24.7 kJ mol(-1), which is in good agreement with the literature value of -19.7 kJ mol(-1). (The weighted average Delta H degrees(f298) for all conformers is -22.6 kJ mol(-1).) Intramolecular interactions in TMEDA such as the steric effect, gauche effect, anomeric effect, and C-H ... N hydrogen bonding were investigated by molecular mechanics and natural bond orbital analysis.