The geometries and force fields of phenylcarbene (PC) and cycloheptatrienylidene (CHT) in their singlet and triplet electronic states as well as of cycloheptatetraene (CHTE) and bicyclo[4.1.0]heptatriene (BCT) and the transition states for the formation and decay of the latter were evaluated by various methods. Relative single point energies were calculated at the CCSD(T)/cc-pVDZ//BLYP/6-31G* level. Finally, the effects of extending the basis set to triple-zeta quality were estimated by (R)MF2 calculations and carried over proportionally to CCSD(T). These calculations show that CHTE which has a strongly distorted allenic structure is the most stable species on that part of the C(CH)(6) surface which was examined in the present study, followed by planar (PC)-P-3. The strained BCT is found to be nearly degenerate in energy with (PC)-P-1, but the high activation energy for its formation from (PC)-P-1 together with the low activation energy for ring-opening to CHTE suggests that this species cannot persist under the experimental conditions employed for production of CHTE. In analogy to the case of cyclopentadienylidene, CHT exists in the form of a closed shell singlet ((1)A(1)) and two related pairs of open shell singlet and triplet states ((1,3)A(2) and B-1.3(1)) which correspond to the Jahn-Teller distorted structures of the cycloheptatrienyl radical, The relative energies and the nature of the different CHT stationary points depend on the method of calculation, but it appears that the decrease in electron repulsion lowers the (1)A(2) State slightly below the (1)A(1) state so that the open shell species serves as a planar transition state for enantiomerization of CHTE with an estimated activation energy of similar to 20 kcal/ mel. The two triplets are very close in energy with the higher lying being either a transition state or a shallow minimum. The B-1(1) state is an excited state of the open-shell singlet. The calculated IR spectra of the three most stable isomers were compared to those published previously by Chapman et al. whereby the assignment of the photoproduct of UV photolysis of phenyldiazomethane to CHTE was confirmed. A full study of the force fields of PC and CHTE is under way.