[en] This paper presents the results of large-scale ab initio calculations including electron correlation effects of the rotation barriers in ethylene, allene and methylene-cyclopropane (MCP). Optimization of the geometry of the method is discussed and results are presented. The authors analyze the correlation contributions to the barrier, and they address the problem of the out-of-plane deformation in MCP and the cycloprypyl radical. The question of whether the singlet or the triplet state is the lowest one in the rotated geometries is discussed