1 Supplementary Material for: Heme protein oxygen affinity regulation exerted by proximal effects Luciana Capece, Marcelo A. Marti, Alejandro Crespo, Fabio Doctorovich and Darío A. Estrin * . Departamento de Química Inorgánica, Analítica y Química Física / INQUIMAE-CONICET, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón 2, Buenos Aires, C1428EHA, Argentina. * To whom correspondence should be addressed. E-mail: dario@qi.fcen.uba.ar , Fax:(5411) 4576 3341 Computational Methodology The quantum subsystem in our scheme is treated at the density functional theory (DFT) level using the efficient SIESTA (Spanish Iniciative for the Electronic Structure of Thousands of Atoms) implementation. 1 The use of standard norm-conserving pseudopotentials 2 avoids the computation of core electrons, smoothing at the same time the valence charge density. We have used the Troullier- Martins scheme with pseudopotential radii for Fe orbitals of 2.3 bohrs, and for all other elements orbitals of 2.0 bohrs. In our study, the nonlinear partial-core correction 3 is applied to the iron atom. Basis functions consisting of localized (numerical) pseudoatomic orbitals, are projected on a real space grid to compute the Hartree potential and exchange correlation potentials matrix elements. For all atoms, basis sets of double plus polarization quality were employed, with a pseudoatomic orbital energy shift of 25 meV and a grid cutoff of 150 Ry. 1,4 The supercell geometry was chosen large enough for the