Relative energy of the high-(T2g5) and low-(A1g1) spin states of [Fe(H2O)6]2+, [Fe(NH3)6]2+, and [Fe(bpy)3]2+: CASPT2 versus density functional theory

Abstract

High-level ab initio calculations using the CASPT2 method and extensive basis sets were performed on the energy differences of the high-[(5)T(2g):t(2g) (4)e(g) (2)] and low-[(1)A(1g):t(2g) (6)] spin states of the pseudo-octahedral Fe(II) complexes [Fe(H(2)O)(6)](2+), [Fe(NH(3))(6)](2+), and [Fe(bpy)(3)](2+). The results are compared to the results obtained from density functional theory calculations with the generalized gradient approximation functional BP86 and two hybrid functionals B3LYP and PBE0, and serve as a calibration for the latter methods. We find that large basis set CASPT2 calculations may provide results for the high-spin/low-spin splitting DeltaE(HL) that are accurate to within 1000 cm(-1), provided they are based on an adequately large CAS[10,12] reference wave function. The latter condition was found to be much more stringent for [Fe(bpy)(3)](2+) than for the other two complexes. Our "best" results for DeltaE(HL) (including a zero-point energy correction) are -17 690 cm(-1) for [Fe(H(2)O)(6)](2+), -8389 cm(-1) for [Fe(NH(3))(6)](2+), and 3820 cm(-1) for [Fe(bpy)(3)](2+).

Embedding Fragment ab Initio Model Potentials in CASSCF/CASPT2 Calculations of Doped Solids: Implementation and Applications

“…Another source of errors is the insufficient basis set on the manganese ion which, in particular, leads to the overestimation of the excitation energies to states with different spin multiplicity. 44 However, this drawback is not expected to affect much the assessment of the FAIMP method for this system.…”

Spin states of (bio)inorganic systems: Successes and pitfalls

“…Also, the use of wavefunction methods is not without hazards, with the ' 'gold standard' ' CCSD(T) proving unreliable for some systems [5] due to the intrinsic multiconfigurational character of the wavefunction. More reliable data were obtained with CASPT2, [5,6] but this method is computationally expensive as well, needs specific expertise and much depends on the choice of orbitals to be included in the active space and the basis set used. For instance, for Fe(H 2 O) 6 2þ , CASPT2 with six electrons in five orbitals [6,5] complete active space (CAS) space with the small 6-31G** basis set gives a [7] which goes down by about 16 kcal mol À1 (to give 46.3 kcal mol) À1 with 12 electrons in 10 orbitals [12,10].…”

“…More reliable data were obtained with CASPT2, [5,6] but this method is computationally expensive as well, needs specific expertise and much depends on the choice of orbitals to be included in the active space and the basis set used. For instance, for Fe(H 2 O) 6 2þ , CASPT2 with six electrons in five orbitals [6,5] complete active space (CAS) space with the small 6-31G** basis set gives a [7] which goes down by about 16 kcal mol À1 (to give 46.3 kcal mol) À1 with 12 electrons in 10 orbitals [12,10]. Using larger basis sets, Pierloot and Vancoillie [6] [8] on Mn-oxo corrole and corrolazine shows differences of up to 5 kcal mol À1 between CASPT2 and RASPT2 for low-lying states (8-15 kcal mol À1 above the ground state), and up to 10 kcal mol À1 for higher ones (31-42 kcal mol À1 above the ground state).…”

“…For instance, for Fe(H 2 O) 6 2þ , CASPT2 with six electrons in five orbitals [6,5] complete active space (CAS) space with the small 6-31G** basis set gives a [7] which goes down by about 16 kcal mol À1 (to give 46.3 kcal mol) À1 with 12 electrons in 10 orbitals [12,10]. Using larger basis sets, Pierloot and Vancoillie [6] [8] on Mn-oxo corrole and corrolazine shows differences of up to 5 kcal mol À1 between CASPT2 and RASPT2 for low-lying states (8-15 kcal mol À1 above the ground state), and up to 10 kcal mol À1 for higher ones (31-42 kcal mol À1 above the ground state). However, one of the most important things to be noted in this latter paper is that even though there are many spin states within a range of 20 kcal mol À1 for the Mnoxo species, [8] both CASPT2/RASPT2 and density functionals (BP86, B3LYP, PBE0, OLYP) agreed very well on the groundstate for Mn-oxo with both the corrole and corrolazine ligands.…”

“…The energy difference between these T 2g and E g orbitals is then usually referred to as the octahedral splitting (D oct , or 10D q ). The magnitude of this splitting depends on the ' 'crystal field' ' strength of the ligands, for example, the Fe(II)(H 2 O) 6 2þ (d 6 ) complex is high-spin ('t 2g ' 4 'e g ' 2 ) with a D oct splitting of about 30 kcal mol À1 , whereas Fe(II)(CN) 6 4À (d 6 ) is low spin ('t 2g ' 6 ) with a D oct splitting of about 94 kcal mol À1 . [1] Spin states play an important role in enzymatic reactions (e.g., cytochrome P450cam, vide infra), in metal-oxo complexes, in spin-crossover (SCO) compounds and there exists even spinstate catalysis where different reactions take place for different spin states.…”