A number of popular density functionals are calibrated against high-resolution EPR data for low spin iron-nitrosyls present in nitric oxide bound myoglobin. The model incorporates both electrostatic and steric effects using a combined quantum mechanics/molecular mechanics (QM/MM) approach. Compared to the most recent experimental data, the calculated EPR parameters using GGA functionals are distinctly more accurate than those predicted by hybrid functionals. The latter is related to an erroneous spin distribution predicted with hybrid functionals in this particular case. However, owing to the inaccuracies in the prediction of spin state energetics, GGA functionals overestimate the binding energy of the nitric oxide ligand to the heme center. Using the calculated magnetic resonance parameters as well as the calculated rotational potential energy surface of the NO ligand over the heme plane, a possible geometric structure of elusive A-form if nitrosyl-myoglobin is proposed.
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