In this research, thermal rearrangements of the Berson trimethylenemethanes (Berson-TMMs) have been investigated by employing density functional theory (DFT) and high-level ab initio methods, such as the complete active space self-consistent field (CASSCF), multireference second-order Moller-Plesset perturbation theory (MRMP2), multireference configuration interaction singles and doubles (MRCISD), and coupled-cluster singles and doubles with perturbative triples [CCSD(T)]. In all computations Pople's polarized triple-zeta split valence basis set, 6-311G(d,p), is utilized. The relevant portions of the lowest-energy, singlet-spin potential energy surface of the C4H6 (parent TMM), C6H8 (Berson-TMMa), and C8H12 (Berson-TMMc) chemical systems have been explored in order to determine the reaction energies and activation parameters accurately, with the ultimate objective of providing a theoretical account of experiments by Berson on TMMc. The nature of the orthogonal and the planar structures of the parent TMM have been clarified in this study. We have concluded that the orthogonal TMM B-1(1) minimum has a C-2v symmetry structure, and there is no pyramidalization in the unique methylene group. It lies at 13.9 kcal mol(-1) above the triplet minimum B-3(2) at MRCISD level. The closed-shell (1)A(1), state of the planar TMM is not a true minimum but a transition structure (TS) for 180 degrees rotation of the unique methylene group in the orthogonal TMM minimum. It lies at 3.0 kcal mol(-1) above B-1(1). The planar structures are also involved in the interchange of equivalent orthogonal TMMs (o(1), o(2), o(3)). Many features of the parent TMM are retained in TMMa and TMMc, despite the constraints imposed by the five-membered ring in the latter species. Thus, ring closure to the bicyclic molecules 3a (3c) and 5a (5c) takes place similarly to that in the parent TMM. Likewise, planar TMMa (TMMc) structures are TSs, while orthogonal ones are true minima. The adiabatic singlet-triplet gaps are also similar, being 14.7 (13.0) and 16.5 (16.2) kcal mol(-1) in the orthogonal (o(1)) and planar TMMa (TMMc), respectively. It has been shown here that the substantial reductions in the ring-opening barriers of MCP derivatives 3a (3c) and 5a (5c) can be largely attributed to ring strain in the former and pi-bond strain in the latter species.