Density functional theory and ab initio quantum mechanical techniques were employed to study the ring opening of singlet cyclopropylidene 1-S and the internal rotation of singlet allene 2-S. The B3LYP, CISD, and CASSCF(4,4) geometry optimizations used a TZP basis set. Single-point TZ2P energies were evaluated at CCSD(T) and multireference configuration interaction with single and double excitations (MR-CISD) levels. Employing the four most important configurations at the MR-CISD/TZ2P//UB3LYP/TZP level gave singlet-triplet separations for 1 of 11.1 kcal mol(-1) and for 2 48.9 kcal mol(-1), Asymmetric transition structures for the ring opening of 1-S were obtained at B3LYP/TZP and CISD/TZP. The barriers are 4.2-4.5 kcal mol(-1) at CCSD(T)/TZ2P with these geometries. In contrast, only one C-s symmetric TS was located at CASSCF(4,4), and the ring opening barrier is only 3.6 and 3.1 kcal mol(-1) at CCSD(T)/TZ2P and MR-CISD/TZ2P, respectively. At CCSD(T)/6-31G* an asymmetric structure for the TS results confirming the predictions of the B3LYP and CISD methods. The B3LYP/6-31G* intrinsic reaction coordinate (IRC) delineated the complex modes of rotation of the methylene groups during the ring opening. Although the overall rotation in going from 1-S to 2-S must be conrotatory, the ring opening of 1-S starts with a disrotatory motion of both methylene groups. However, before the C-1 TS is reached, the sense of rotation of one of the methylene groups changes. At the B3LYP and CASSCF(4,4) levels, the transition state for the internal rotation of allene is a planar bent (C-2v) open-shell singlet ((1)A(2)) The D-2h open-shell singlet is a higher lying second-order stationary point. All D-2h and C-2v closed-shell species are significantly higher in energy. MR-CISD single-point activation barriers obtained from the B3LYP and CASSCF(4,4) geometries were 44.6 and 45.5 kcal mol(-1), respectively. The B3LYP/TZP ring opening (4.8 kcal mol(-1)) and allene internal rotation (44.6 kcal mol(-1)) barriers are in remarkably good agreement with the CCSD(T)/TZ2P//B3LYP/TZP value (4.5 kcal mol(-1)) on the barrier and with the MR-CISD/TZ2P//B3LYP/TZP barrier for the latter (44.6 kcal mol(-1)).