= Solution
The <molecular orbital> diagram has one level at $\alpha+2\beta$ labelled $A_{1g}$, two degenerate levels at $\alpha$ labelled $E_u$, and one level at $\alpha-2\beta$ labelled $B_{1g}$, in increasing <energy> for $\beta<0$. Each spatial <molecular orbital> admits two opposite-spin <electrons>. The diagram below also shows how one representative distortion lifts the middle degeneracy; the distorted scale is illustrative, not a fitted molecular spectrum.
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-2b-huckel-levels.png]
{title=Square-array orbital levels and their splitting in a representative trapezium model}
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