The structure of 1,3 dinitrobenzene radical anion in the doublet ground and lowest excited states was studied by ab initio multiconfiguration CASSCF methods. The results of calculations suggest the existence of one symmetrical and two asymmetrical structures of the radical anion. The energies of these structures were estimated.Key words: radical anion, 1,3 dinitrobenzene, intramolecular electron transfer, ab initio quantum chemical calculations, CASSCF method.The results of experimental and theoretical studies show that transfer of an electron to a neutral molecule resulting in a radical anion is accompanied by changes in geometric parameters. 1 A particular case is the electron transfer to a symmetrical molecule followed by formation of several isomeric asymmetrical radical anions that are in fast dynamic equilibrium. This phenomenon, which mani fests itself in the broadening of spectral lines, was ob served 2 in ESR studies of radical anions of aromatic dinitro derivatives.The available theoretical interpretations of this phe nomenon are ambiguous. One of them treats the asym metrical geometry of radical anions formed from sym metrical neutral molecules as a result of either asym metrical specific solvation of such molecules 3 or higher stabilization (compared to symmetrical structures) of the asymmetrical structure due to non specific solvation. 4 An alternative concept 2 assumes an asymmetrical structure of isolated radical anion. To confirm this assumption, ab initio UHF и MP2 calcuations of 1,3 dinitrobenzene (DNB) radical anion were carried out, 2 but the calculated spin densities and S 2 values were found to be incorrect and the authors had to use the AM1 semiempirical method. However, they failed to obtain unambiguous re sults 2 because the energy difference between the sym metrical and asymmetrical forms of the DNB radical an ion was too small to state with certainty that isolated DNB radical anion has an asymmetrical structure.The electronic structure of radical anions of aromatic nitro derivatives is of interest for both basic and applied chemical research. 4 Therefore, we carried out an ab initio multideterminant quantum chemical study of the elec tronic structure of DNB radical anion. The electronic structure of radical anions of aromatic compounds has an antibonding molecular orbital (МО) occupied by an elec tron; therefore, one should expect that in this case the energy differences between the highest occupied and low est unoccupied МОs will be much smaller than in the starting neutral molecule and, hence, the effects of con figuration interaction witl be much more pronounced.

A brief survey of the state of the art in methods of calculations of protein-ligand interaction energies in docking complexes is presented. A new computational technique is proposed that allows one to fundamentally improve the performance of large scale serial calculations of docking complexes using the AM1/PM3 semiempirical methods. The technique explicitly al lows for a specific feature of docking problems, viz., the need for calculating numerous ligand complexes with a specified protein whose noninteracting part remains "frozen" during computa tions. The interaction energies calculated using the new method differ only slightly from the results of complete АМ1 calculations and the performance attained is high enough to solve practical drug design problems.Key words: large scale calculations of docking complexes, fast quantum chemical calcula tions, semiempirical methods.The determination of activity of relatively small ligand molecules (prototypes of active principles of drugs) in the interaction with large protein molecules is a key problem in biochemistry. Knowledge of energies of the interaction of a protein with ligands for very large ligand collections is of crucial importance for efficient targeted drug design. Meanwhile, the design of pharmaceuticals including high ly specific and even individually "tuned" ones (in the fu ture) belongs to scientific challenges of crucial importance.It is appropriate to begin large scale preliminary screening of ligands to choose best candidates from the standpoint of ligand-protein interactions with theo retical studies using computer simulation followed by more expensive and long term experiments with the ligands chosen.The increasing use of computer assisted docking* is due to important role of this technique for novel drug de sign technologies. The demand of rapidly developing phar maceutical industry for less expensive drug design, which becomes increasingly more complicated 1 against the back ground of exponentially increasing computer performance, makes the docking (or, more generally, computer assisted * From this point on we use the term "docking" in a broad sense including the determination of ligand arrangement in the protein cavity and the energy of the protein-ligand interaction.

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The structure of the 1,3 dinitrobenzene dianion in the ground and lowest excited electron states was studied by quantum chemical methods. The dianion, unlike the radical anion, is characterized by the symmetrical structure in both the ground triplet ( 3 B 1 ) and lowest excited singlet ( 1 A 1 ) states. The wave function of the singlet state has the biradical character to a great extent. The singlet triplet splitting calculated by the CASSCF and MRMP2 methods is 6 and 2 kcal mol -1 , respectively.

The fundamentally new method NESE is proposed for quantum chemical calculations of large molecules, which employs the approximation of the Hamiltonian of the commonly used DFT method and is as fast as the AM1 and PM3 semi empirical methods or the DFTB method. The parameters for the new method were chosen by the least squares method based on the comparison of its matrix elements with the reference DFT/PBE Hamiltonian. The initial non iterative version NESE 0 was computer implemented and approved on many thousands of various molecules containing H, C, N, and O atoms. The NESE 0 method moderately outper forms the DFTB approach and is an order of magnitude better than the AM1, PM3, and PM6 levels in reproducing the one electron energies calculated in terms of the DFT/PBE. Key words: approximate DFT Hamiltonian, fast quantum chemical methods, large mole cules, selection of parameters.The computer modeling of large biological molecules and nanostructures at the quantum chemical level allows researchers to reduce the time of the design of such ob jects, as well as the time of obtaining adequate informa tion on the quantum electronic properties (as opposed to those obtained by molecular mechpanics) and the spatial structures of the newly designed and the already available bio and nanostructures. Experiments for such structures become more and more expensive, while the cost of calcu lations being rapidly reduced. This promising field of high performance calculations is very important, for example, in solving problems of the computer drug design. In the latter case, it is necessary to perform calculations of giant molecules, which requires large computer resources even when employing commonly used and reasonably accurate density functional theory (DFT) methods, not to mention more precise and sophisticated approaches. Hence, semi empirical methods, such as AM1, 1 PM3, 2 and PM6, 3 as well as the approximate DFT method SCC DFTB (self consistent charge density functional tight binding) meant for fast calculations of large systems, 4 were em ployed as alternative approaches.Earlier, we have achieved the record speed for a num ber of simple semi empirical quantum chemical meth ods. 5,6 This allows one to perform large scale calculations (thousands of complexes, each consisting of thousands of atoms) using standard computers. However, the need arises for fast calculations with an accuracy of modern DFT methods widely used in far less large scale calculations. In the present work, we propose a new quantum chemical method for calculations of large molecules, which is al * On the occasion of the 80th anniversary of the N. D. Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences.