The currently available computational methods have often serious limitations to treat systems where electron correlation plays and important role. Many issues concerning the electronic structure of radicals, photoreceptors near-half-filled transition metals (Cr,Mo,Fe,Ni) are of paramount relevance in basic and applied research in Chemistry and Biochemistry, but still out of the capabilities of standard and conventional tools such as Density Functional Theory. On the other hand, post Hartree-Fock methods computationally more expensive and their application is limited to few atoms. The objective of the present proposal is to overcome these limitations and to develop and apply a multiscale, innovative and unconventional computer simulation technique to unravel the electronic properties of strongly correlated chemical and biochemical systems. The methodology is based on a combined approach between Quantum Monte Carlo (QMC), DFT and Molecular Mechanics. The proposed approach has a faster scaling of the calculation time with the system size N with respect others standard quantum chemistry methods of equivalent level (~ N4 vs ~ N7). es to address challenging open problems in the chemistry and biochemistry of radical compounds, photoreceptors, and transition metal catalysis and enzymatic activity. Application to photoreceptors include the study of the spectral properties of rhodopsin, the integral membrane protein responsible of the light detection in the retina. Applications on transition metal molecules will shed the light on the catalytic strategies of iron-based enzymes and their corresponding biomimetic compounds.