The (X) over tilde (2)A(1) and (a) over tilde (4)A(2) electronic states of the methylene cation, CH2+, were investigated using the coupled cluster method with singles, doubles, and perturbatively applied triples [CCSD(T)] with Dunning's correlation consistent polarized valence basis set series (cc-pVXZ, where X = T, Q, and 5), core-valence basis sets (cc-pCVXZ, where X = T and Q), and augmented basis sets (aug-cc-pVXZ, where X = Q and 5). Explicit computation of the full set of triples (CCSDT) was also performed with the cc-pVTZ basis set. The most reliable equilibrium structures of r(e) = 1.094 Angstrom and theta(c) = 140.4degrees ((X) over tilde (2)A(1)) and r(e) = 1.190 Angstrom and theta(c) = 77.1degrees ((a) over tilde (4)A(2)) were obtained at the CCSD(T)/aug-cc-pV5Z level. The (X) over tilde (2)A(1)-(a) over tilde (4)A(2) classical energy separation is predicted to be 86.9 kcal/mol (30400 cm(-1), 3.77 eV) at the CCSD(T)/cc-PCVQZ level of theory, and the zero-point vibrational energy corrected value is 84.5 kcal/mol (29500 cm(-1), 3.66 eV). (C) 2002 Published by Elsevier Science B.V.