We first demonstrate that our method is capable of capturing the inverted Δ𝐸 ST for some model compounds. As shown by Toyota and co-workers, pentalene at the ideal 𝐷 2h geometry has an inverted gap, while relaxation to the minimum with 𝐶 2h symmetry (using B97-3c) leads to a normal gap. In our calculations, pentalene with equal bond lengths of 1.4 Å displays a small HOMO-LUMO overlap of 0.24, leading to a small exchange interaction of only 2𝐾 = 0.130 eV (Figure ). The dynamic spin polarization correction Δ𝐸 DSP SCF of -0.389 eV leads to a net predicted Δ𝐸 DSP ST,SCF of -0.259 eV. Adding additional correlation with CIS leads to a Δ𝐸 DSP ST,CIS of -0.177 eV. The simple perturbation theory model allows us to gain additional insight into the contributions to Δ𝐸 DSP SCF as they correspond to single excitations from doubly occupied orbitals below the HOMO to unoccupied orbitals above the LUMO (Figure ). Three excitations, HOMO-3 → LUMO+2, HOMO-2 → LUMO+1 and HOMO-1 → LUMO+3 (1 → 7, 2 → 6 and 3 → 8 in Figure ) are identified as the main contributors to the spin polarization effect, and could potentially be tuned by substituent effects.