Chemical structure parameters have been calculated by the RHF/6-31G** ab initio method for model polycyclic molecular fragments of petroleum asphaltenes. A nonplanar character of the planes with the dihedral angle alpha between the planes of the aromatic and naphthenic moieties in the range of 154A degrees-164A degrees has been revealed for these entities. It has been shown that the stacking of polycyclic structures is facilitated by the possibility of donor-acceptor interactions.