Abstract. We present an efficient and accurate method for calculating XAFS Debye-Waller factors for deformed active sites of hemoproteins and metal substituted hemes. Based on the Normal Coordinate Structural Decomposition scheme, the deformation of the porphyrin macrocycle is expressed as a linear combination of the normal modes of the planar species. In our approach, we identify the modes that contribute most to the deformation. Small metal-porphyrin structures which match the macrocycle structural deformation of the deformed hemoprotein site are used to calculate the Debye-Waller parameters at sample's temperature. The Debye-Waller factors are directly obtained by calculating the normal mode spectrum of the corresponding metal-porphyrin structure using Density Functional Theory. Our method is tested on Ni-tetraadamantyl porphyrin and cytochrome c structures with more than 500 available scattering paths.