Theoretical Considerations of Tetramethyleneethane

The triplet state of 33 can be described by a single configuration. The state, using the D2J designation, has the wavefunction 

The singlet state will require at least two configurations since the NBMOs (MO 22 and 23) are close in energy. Thus, a reasonable reference wavefunction for 

Any computation that does not account for the multiconfigurational nature of singlet TME will run into difficulties, analogous to the difficulties discussed previously with methylene and phenylnitrene and with the benzynes discussed later in this chapter. 

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