Table 1.

Energies (E) of various model systems in different spin states (in kcal/mol)

Models	E	Models	E
*	16.1	O1	0.2
	28.2	O2	0
	19.4	O3	3.8
	5.0	F1/2	0
	5.0	F3/2	0
	17.8	F5/2	14.71
	6.9		0
	29.2		25.1
	13.8		9.2
	2.5		1.7
	14.6		24.3
	47.8		0

The energies (E) reported in the table are obtained at the B3LYP (Becke, three-parameter, Lee-Yang-Parr)/def2–TZVP (triple zeta valence plus polarization)/dielectric constant ε = 4 level of theory. The energies of all O_H and O, F_H ,and F, O_H,R and O_R, and F_H,R and F_R spin states are relative to the corresponding lowest energy O, F, O_R, and F_R states (i.e., O², F^1/2, , and ).

^*The H subscript in O_H state denotes the hydroxyl ligand of heme a₃ Fe (see The Metastable O_H State). For all other states, subscript H is retained for consistency and to distinguish them from “inactivated” four-coordinated states (O, F, O_R, and F_R).