Fe(CO)₄ flexible as a two‐level system avoided conical intersection

Vinylidene carbonylation at a manganese–iron complex: A density functional study of mechanism

“…There is no experimental estimate of the energy difference between these two states [25]. Theoretical works report a variety of values for singlet-triplet gap, from 0.5 kcal/mol to 20 kcal/mol ( [26,27] and references therein), depending on the type of computational method and the basis sets used. Our B3LYP calculated value, 9.8 kcal/mol, falls in the middle of this interval.…”