[QUE/QM-23010]

A hydrogen atom is subjected to perturbation $V_\text{pert}(r)=\epsilon \cos(2r/a^0)$, where \(a_0\) is the Bohr radius. The change in the gorund state energy to first order in \(\epsilon\) is

1. \(\epsilon/4\)  
2. \(\epsilon/2\)
3. \(-\epsilon/2\)
4. \(-\epsilon/4\)