The planar structure of cis-butadiene is often discussed in textbooks as the conformation of butadiene that will undergo the Diels-Alder reaction. There has been considerable discussion in the past as to whether the planar cis-structure is a minimum or whether the structure is actually a gauche-structuren (e.g. twisted around the C2-C3 bond by a few degrees). The issue is not completely settled at this time. So, cis-butadiene could be a transition state structure for rotation about the C2-C3 bond between the gauche-structures. Or, alternatively, the cis-structure could be a local minimum. The cis-structure is advantageous as it allows for conjugation, but this conformation has the disadvantage of steric interactions between the terminal methylene groups. At the MP2(full)/6-31G* level, the cis-structure is a minimum.