Density functional computations on a series of complex systems and processes are presented and discussed. In particular the following subject have been investigated: i) catalytic mechanism of carbonic anhydrase; ii) structural properties of silver mediated DNA dimers; iii) structural and spectroscopic features of Zn-phthalocyanine derivatives; iv) carnosine-carboplatin, carnosine-oxaliplatin complexes fragmentation pathways. Reported data indicate that good results can be obtained selecting the appropriate computational strategies, exchange-correlation functionals and basis sets.