The additivity of the pi-electron correlation energy in planar heteroatomic molecules

We have shown that the It-electron correlation energy of planar chain and b ranched polyenes containing various heteroatoms follows simple additivity r ules. It is a multilinear function of a number of atoms of each element ent ering into a given compound. The same holds for the nondynamical E(ND)(pi) and dynamical E(D)(pi) components of the total correlation energy. It is fo und that the former is insensitive to the quality of the employed basis set . In contrast, the dynamical correlation is strongly dependent on the intri cacy of utilized basis sets. These findings are rationalized by taking into account a difference in their definitions, the different nature of E(ND)(p i) and E(D)(pi) correlation energies, and the way of their calculation. An extension of the completely active subspace considerably changes E(ND)(pi) and E(D)(pi) values and to some extent introduces changes in the total pi - electron correlation. However, the additivity rule persistently holds, indi cating that it is a robust property. Finally, it is shown that the E(ND)(pi ) energy can serve as a good indicator of the intrinsic (anti)aromaticity o f cyclic compounds. (C) 2001 Elsevier Science B.V. All rights reserved.