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The electronic partition function includes electronic excitation energies and degeneracies $g_i$. If only an exactly degenerate ground manifold of size $g$ is populated, $q_{\mathrm{el}}=g$ and its molar entropy is $R\log g$. Resolved splittings require a state sum rather than replacing all components by one degeneracy.

Set the ground electronic energy to zero and write the excitation energy as $\Delta =hc(120\mathrm{c}{\mathrm{m}}^{-1})$, or the characteristic temperature $\theta =\Delta /k_{\mathrm{B}}=172.65$ K. Counting each doubly degenerate manifold once gives the single-molecule electronic partition functionFor $N$ independent molecules the electronic factor is $q_{\mathrm{el}}^N$. Hence the electronic internal energy isThe equal degeneracies cancel from the excited-state probability. At low temperature the ground manifold dominates and $U_{\mathrm{el}}\to 0$; at high temperature both manifolds have equal populations, giving $U_{\mathrm{el}}\to N\Delta /2$. These are electronic contributions only, excluding translation, rotation and vibration.

If the relevant ground manifolds have electronic degeneracies $g_A$, $g_B$ and $g_{\mathrm{TS}}$, and their components are treated as energetically degenerate, multiply the result byOnly reactive transition-state electronic channels should be counted. Resolved electronic splittings require electronic partition functions rather than simple integer degeneracies.