The ring-opening reaction of singlet cyclopropylidene to singlet D(,2d) allene is studied by means of ab-initio FORS-MCSCF calcula- tions. The energy surface is determined for many intermediate geometries defined by the C-C-C opening angle and the two CH(,2) rotation angles. For each choice of these three variables, the remaining twelve internal coordinates are relaxed by energy minimization. The results obtained differ markedly from those of earlier, less sophisticated calculations. As the C-C-C angle opens, the reaction is found to proceed first on a disrotatory pathway uphill to the transition region around a C-C-C angle of 84(DEGREES), where the reaction path bifurcates. It then follows one of two more or less conrotatory downhill pathways. From an opening angle of about 100(DEGREES) on there exists a state of free synchronized disrotatory motion in the molecule. It is shown that the reaction of the unsubstituted species is non-stereospecific;The characteristics of the bifurcating transition region, the first one ever found for a chemical reaction, are examined in detail and the theory governing similar surfaces is developed and generalized. It is shown that for a surface to exhibit such a bifurcation, a Valley Ridge Inflection (VRI) point must be present, i.e. the reaction path valley must, at some point, change into a ridge. Several examples of such surfaces are given and the actual reaction surface is fitted to the appropriate model by means of an analytical least mean squares fit;The geometries are first calculated using a minimal basis set. Based on these geometries, extended basis set calculations (including polarization functions) are performed for key points along the reaction path. The main effect of this basis set improve- ment is a substantive lowering of the ring-opening barrier;Since experimental evidence, based exclusively on the substi- tuted species, suggests that the reaction is stereospecific, a potential simulating the steric effects of substituents is added to the originally calculated surface. The results are analyzed and it is shown that all;experimental observations can be adequately explained solely by means of nonbonded interactions between substituent groups; *USDOE Report IS-T 1212. This work was performed under Contract No. W-7405-eng-82 with the U.S. Department of Energy.