High-quality ab initio quantum chemical methods, including higher-order coupled cluster and full configuration interaction benchmarks, with basis sets ranging from [C/H] [4s3p1d/2s1p] to [9s8p7d5f4g3h2i/7s6p5d4f3g2h] have been employed to obtain the best technically possible value for the standard enthalpy of formation of (X) over tilde B-3(1) CH2 and (a) over tilde (1)A(1) CH2. Careful extrapolations of finite basis MP2, CCSD, CCSD(T), and CCSDT energies to the complete basis set full configuration interaction limit plus inclusion of small corrections owing to relativistic effects, core correlation, and the diagonal Born-Oppenheimer correction results in the final extrapolated enthalpies of formation of this study, Delta(f)H(0)(o)( (X) over tilde B-3(1) CH2) = 390.45(-0.64)(+0.68) kJ mol(-1) and Delta(f)H(0)(o)((a) over tilde (1)A(1) CH2) = 428.10(-0.64)(+0.68) kJ mol(-1). The computed value for <()over tilde> B-3(1) CH2 is in between the best two experimental results of 389.87+/-0.86 and 390.73+/-0.66 kJ mol(-1). The elaborate calculations leading to these enthalpies of formation also resulted in accurate estimates of the singlet-triplet splitting, T-0((a) over tilde (1)A(1) CH2) = 37.54(-0.29)(+0.41) kJ mol(-1), in excellent agreement with the best empirical value of 37.65+/-0.06 kJ mol(-1), of the total atomization enthalpy, D-0((X) over tilde B-3(1) CH2) = 753.03(-0.62)(+0.43) kJ mol(-1), in excellent agreement with the best experimental value of 753.3 kJ mol(-1), of the bond dissociation energy, D-U1(o)(0 K)(CH-H) = 417.85+/-0.35 kJ mol(-1), and of the quartic force field representations of the potentials of the two states around their respective minima. (C) 2003 American Institute of Physics.