Past exam of the chemistry A course of the University of Cambridge / 2022 / Paper 1 / A4 / c / i / 2 / Solution

For ${\beta }_L=0$, four isolated short-bond dimers each contribute $2(\alpha +{\beta }_S)$:These are different coupling limits. Keeping ${\beta }_S=\beta$ while simply removing long-bond coupling does not represent a small distortion at fixed mean bond strength.