Since only the topology of the carbon framework is of significance in the HMO method, the full symmetry of s-trans-butadiene need not be used to get the maximum simplification possible in the HMO method; instead, it is sufficient to use only the C2 axis.
(a) Write down the two possible symmetry species for the group C2.
(b) Construct π-electron symmetry orbitals for butadiene, classifying them according to the symmetry species of C2.