5 , ▁ 1 4 . 7 ▁and ▁ 1 2 . 6 ▁e V . ▁As ▁explained ▁above , ▁the ▁devi ations ▁are ▁due ▁to ▁the ▁effects ▁of ▁orb ital ▁relax ation ▁as ▁well ▁as ▁differences ▁in ▁electron ▁correlation ▁energy ▁between ▁the ▁mole cular ▁and ▁the ▁various ▁ion ized ▁states . ▁ ▁For ▁N 2 ▁in ▁contrast , ▁the ▁order ▁of ▁orb ital ▁energ ies ▁is ▁not ▁identical ▁to ▁the ▁order ▁of ▁ion ization ▁energ ies . ▁Near - H art ree – F ock ▁calculations ▁with ▁a ▁large ▁basis ▁set ▁indicate ▁that ▁the ▁ 1 π u ▁bond ing ▁orb ital ▁is ▁the ▁H OM O . ▁However ▁the ▁lowest ▁ion ization ▁energy ▁corresponds ▁to ▁removal ▁of ▁an ▁electron ▁from ▁the ▁ 3 σ g ▁bond ing ▁orb ital . ▁In ▁this ▁case ▁the ▁deviation ▁is ▁attributed ▁primarily ▁to ▁the ▁difference ▁in ▁correlation ▁energy ▁between ▁the ▁two ▁orbit als . ▁ ▁For ▁electron ▁aff in ities ▁ ▁It ▁is ▁sometimes ▁claimed ▁that ▁Ko op mans ' ▁theorem ▁also ▁allows ▁the ▁calculation ▁of ▁electron ▁aff in ities ▁as ▁the ▁energy ▁of ▁the ▁lowest ▁un occup ied ▁mole cular ▁orbit als ▁( L UM O ) ▁of ▁the ▁respective ▁systems . ▁However , ▁Ko op mans ' ▁original ▁paper ▁makes ▁no ▁claim ▁with ▁regard ▁to ▁the ▁significance ▁of ▁eigenvalues ▁of ▁the ▁F ock ▁operator ▁other ▁than ▁that ▁corresponding ▁to ▁the ▁H OM O . ▁Nevertheless , ▁it ▁is ▁straightforward ▁to ▁general ize ▁the ▁original ▁statement ▁of ▁Ko op mans ' ▁to ▁calculate ▁the