  We propose a mechanism for binding of diatomic ligands to heme based on a
dynamical orbital selection process. This scenario may be described as bonding
determined by local valence fluctuations. We support this model using
linear-scaling first-principles calculations, in combination with dynamical
mean-field theory, applied to heme, the kernel of the hemoglobin metalloprotein
central to human respiration. We find that variations in Hund's exchange
coupling induce a reduction of the iron 3d density, with a concomitant increase
of valence fluctuations. We discuss the comparison between our computed optical
absorption spectra and experimental data, our picture accounting for the
observation of optical transitions in the infrared regime, and how the Hund's
coupling reduces, by a factor of five, the strong imbalance in the binding
energies of heme with CO and O_2 ligands.
