The need for theoretical modeling in biological and medicinal chemistry stems from many reasons. Experimental results are often difficult to interpret and the assignment of the observed signals to certain species is arbitrary in many cases. Theoretical models give direct insight into the electronic, energetic and geometric properties of the chemical systems (molecules and their aggregates), and processes they undergo, These then allow relating the obtained quantities with the measured ones. Direct investigation of materials of biological importance by modern experimental techniques is usually hardly possible, at most time- and funds consuming processes. Computational methods allow investigating a variety of hypothetical structures, where the most promising ones may be selected for further study. In addition, unstable or extremely short-living species, crucial for overall processes but being hardly proven by experiments, may be still investigated by theoretical tools.