Effective simulation of biological systems: Choice of density functional and basis set for heme-containing complexes

Using ferric-S-methyl-porphyrin as the prototype of heme-containing complexes, we address how to accurately and efficiently carry out density functional studies for systems of biological importance. Our results indicate that GGA and meta-GGA functionals bias the lower spin state by producing levitated frontier orbitals and often fail to provide a correct description of the ground spin state. Also, we propose composite double and triple zeta quality basis sets with polarization functions applied only to the transition metal and electronegative atoms. Accurate structural and electronic properties including DFT reactivity indices have been obtained and significant gain in computational efficiency has been achieved.