Methylene electron correlation approach

It is a short step from acetylene to ethylene, but instead of constructing a correlation diagram for addition of H2 across the acetylenic triple bond, let us consider the dimerization of two coplanar methylene molecules. For consistency with the axis convention of Figs. 4.1-4.4, the methylene molecules on the the left side of Fig. 4.7 are placed in in the zx plane and allowed to approach each other along their common C2 axis (x), leaving the Py orbitals free for tt bonding. The symmetry of the field exerted on the electrons by the nuclear frame is D2/1 no hypothetical external field need be postulated. 

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