The complementary “lock and key” patterns of weakly interacting molecules are explored by mapping the topography of respective molecular electrostatic potentials (MESP). A new model, viz., electrostatic potential for intermolecular complexation (EPIC), which incorporates such MESP features, has been employed for studying interactions between DNA base pairs. A wide variety of pairs of bases involving adenine (A), guanine (G), and cytosine (C) are chosen as test cases. The interaction energy within the EPIC model is expressed as (1/2){∑V A, i q B, i + ∑V B, i q A, i }, where V A, i is the MESP value due to A at the ith atom of molecule B and q B, i is the MESP-derived charge at this site. The interaction energies and geometrical parameters obtained by this model agree remarkably well with the corresponding literature values obtained by full geometry optimization at the ab initio level. Being intuitively appealing and simple in application, the EPIC model seems to have the potential of being a good predictive tool for investigating weak intermolecular interactions.

Triplexes involving major groove binding of a third oligomer to the DNA duplex structure have gathered much interest in recent years. The study of base trimer interactions in the gas phase is expected to provide useful information regarding orientational preferences and inherent stabilities. A recently developed electrostatic potential for intermolecular complexation (EPIC) model has been found to be quite useful for exploring the structures and energetics in base pairs [Gadre, S. R.; Pundlik, S. S. J. Phys. Chem. 1996, 101, 3298]. This model makes use of complementary electrostatic features of the interacting species that are determined by ab initio theory. We report here the investigations on various trimers including TAT, TAG, ATG, CGG, and TCG using this model. The overall good agreement of the trimer interaction energies with the corresponding single-point SCF values made at the model-predicted geometries reveals the suitability of the EPIC model for studying DNA base complexes.

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We determine the structure, energetics, and emerging magnetic propertiesof Au n clusters using first-principles plane-wave density functional theory. We compare and contrast the findings obtained with and without spin-orbit interaction in the gold clusters of sizes down to 0.5-1 nm. The shapes are chosen to be representative of spherical gold nanoparticles: (a) Au 13 and Au 12 , with cuboctahedral, icosahedral, and decahedral structures, and (b) Au 25 and Au 24 , with truncated octahedral structures. We find that the trends in the binding energies are unaltered with the choice of the exchange correlation (LDA or GGA) or the inclusion of relativistic effects. The cuboctahedral Au 13 is found to have the lowest binding energy compared with others at a given level of theory. Within the scalar relativistic description, these gold clusters exhibit a wide variety of magnetic moments: the stability and magnetic properties can be readily understood in terms of degeneracies of the HOMO and LUMO levels. Further, there is evidence of Jahn-Teller activity in the cases of cuboctahedral Au 12 and Au 13 that leads to structural distortion, inducing magnetism in the 12-atom cluster. Analysis of electronic states with projection on atomic orbitals for the scalar relativistic case shows that the magnetism in these gold clusters has an sp rather than the otherwise believed s character. By employing a fully relativistic description with inclusion of spin-orbit interaction and noncollinear magnetization, the magnetic moment in the icosahedral-shaped clusters is found to reduce substantially and that in the 12-and 13-atom clusters with a cuboctahedral structure becomes vanishingly small. The loss of magnetism in Au 12 and Au 13 appears to originate from the splitting of degenerate HOMO states in these clusters and an overlap of the d and sp states, whereas the magnetic moment of around 1 μ B in Au 25 is mainly caused by the s states of the central atom.

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The conventional electrostatic charge models (PD-AC) are constructed so as to reproduce the molecular electrostatic potential (MESP) on and beyond the van der Waals' (vdW) surface. The MESP distribution has recently IS R Gadre, S A Kulkarni and I H Shrivastava (1992) J. Chem. Phys. 96 5253] been shown to exhibit rich topographical features. With this in view, a detailed topographical comparison of the MESP derived from the charge models, with the respective ab initio (MO) ones is taken up for water, hydrogen sulphide, methane and benzene molecules as test cases. It is shown that the point charge models have a fundamental lacuna, viz. they fail to mimic the essential topographical features of MESP. A new model incorporating a small number of floating spherical Gaussians is shown to restore all the critical features of the molecules under study. A comparative study of the standard deviations of MESP derived from charge models on scaled vdW surfaces further reveals that the present model leads to a better representation of ab initio MESP.