The results of ab initio calculations on 1,3,5-trimethylenebenzene (1) and its negative ion (1(-)) are presented. Geometries were optimized at the CASSCF/6-31G* and CASSCF/6-31+G* levels. Single-point calculations were performed using second-order perturbation and multireference configuration interaction (MR-CI) methods, in order to include the effects of dynamic correlation between the sigma and pi electrons. The ground state of 1 is predicted to be the high spin (4)A(1) " state, which has D-3h symmetry. The lowest energy excited state is E-2 " in D-3h symmetry and is thus subject to first-order Jahn-Teller distortions to geometries of lower symmetry. The C-2v geometries of the two Jahn-Teller distorted components of E-2 ", B-2(1) and (2)A(2), have been optimized and are found to have nearly the same energies, After correction for zero-point energy differences, the adiabatic energy separation between the lowest C-2v doublet and the D-3h quartet ground state of a is computed to be 14 +/- 1 kcal/mol. The ground state of 1(-) is predicted to be E-3’ In D-3h symmetry. Molecular distortion to C-2v symmetry stabilizes the B-3(2) component of E-3’ via not only first- but also second-order Jahn-Teller effects. Consequently, the B-3(2) component of 3E’, at its optimized geometry, is calculated to be lower in energy by ca. 1.5 kcal/mol than the (3)A(1) component at its optimized geometry. The lowest singlet excited state of 1(-) is (’)A(1)’ in D-3h symmetry, which is predicted to undergo second-order Jahn-Teller distortions, at least at the CASSCF level of theory. The resulting state, (1)A(1) in C-2v symmetry, is calculated to be 4-6 kcal/mol above the B-3(2) groundstate but slightly below a (3)A(2)’ excited state of D-3h symmetry. In contrast to CASSCF and MR-Cl, UB3LYP/6-31+G* calculations predict (3)A(2)’ to be the ground state of 1(-) slightly below either component of the Jahn-Teller distorted E-3’ State. The UB3LYP calculations afford an estimate of 21-22 kcal/mol for the electron affinity of H and provide vibrational frequencies for the Ar-4(1)" state. On the basis of computational results for 1 and 1(-), the most important features of the photoelectron spectrum of 1(-) are predicted.