In this study we have investigated the interaction of phenylalanine (Phe), histidine (His), tyrosine (Tyr), and tryptophan (Tryp) molecules with graphene and single walled carbon nanotubes (CNTs) with an aim to understand the effect of curvature on the non-covalent interaction. The calculations are performed using density functional theory and the Moller-Plesset second-order perturbation theory (MP2) within linear combination of atomic orbitals-molecular orbital (LCAO-MO) approach. Using these methods, the equilibrium configurations of these complexes were found to be very similar, i.e., the aromatic rings of the amino acids prefer to orient in parallel with respect to the plane of the substrates, which bears the signature of weak pi-pi interactions. The binding strength follows the trend: His

The geometric and electronic structure of the Pb n clusters ͑n =2-15͒ has been calculated to elucidate its structural evolution and compared with other group-IV elemental clusters. The search for several low-lying isomers was carried out using the ab initio molecular dynamics simulations under the framework of the density functional theory formalism. The results suggest that unlike Si, Ge, and Sn clusters, which favor less compact prolate shape in the small size range, Pb clusters favor compact spherical structures consisting of fivefold or sixfold symmetries. The difference in the growth motif can be attributed to their bulk crystal structure, which is diamond-like for Si, Ge, and Sn but fcc for Pb. The relative stability of Pb n clusters is analyzed based on the calculated binding energies and second difference in energy. The results suggest that n = 4, 7, 10, and 13 clusters are more stable than their respective neighbors, reflecting good agreement with experimental observation. Based on the fragmentation pattern it is seen that small clusters up to n = 12 favor monomer evaporation, larger ones fragment into two stable daughter products. The experimental observation of large abundance for n = 7 and lowest abundance of n = 14 have been demonstrated from their fragmentation pattern. Finally a good agreement of our theoretical results with that of the experimental findings reported earlier implies accurate predictions of the ground state geometries of these clusters.

The geometric and electronic structures of the Pbn+ clusters (n=2-15) have been investigated and compared with neutral clusters. The search for several low-lying isomers was carried out under the framework of the density functional theory formalism using the generalized gradient approximation for the exchange correlation energy. The wave functions were expanded using a plane wave basis set and the electron-ion interactions have been described by the projector augmented wave method. The ground state geometries of the singly positively charged Pbn+ clusters showed compact growth pattern as those observed for neutrals with small local distortions. Based on the total energy of the lowest energy isomers, a systematic analysis was carried out to obtain the physicochemical properties, viz., binding energy, second order difference in energy, and fragmentation behavior. It is found that n=4, 7, 10, and 13 clusters are more stable than their neighbors, reflecting good agreement with experimental observation. The chemical stability of these clusters was analyzed by evaluating their energy gap between the highest occupied and lowest unoccupied molecular orbitals and adiabatic ionization potentials. The results revealed that, although Pb13 showed higher stability from the total energy analysis, its energy gap and ionization potential do not follow the trend. Albeit of higher stability in terms of binding energy, the lower ionization potential of Pb13 is interesting which has been explained based on its electronic structure through the density of states and electron shell filling model of spherical clusters.

A systematic theoretical study of the PbnM (M=C, Al, In, Mg, Sr, Ba, and Pb; n=8, 10, 12, and 14) clusters have been investigated to explore the effect of impurity atoms on the structure and electronic properties of lead clusters. The calculations were carried out using the density functional theory with generalized gradient approximation for exchange-correlation potential. Extensive search based on large numbers of initial configurations has been carried out to locate the stable isomers of PbnM clusters. The results revealed that the location of the impurity atom depends on the nature of interaction between the impurity atom and the host cluster and the size of the impurity atom. Whereas, the impurity atoms smaller than Pb favor to occupy the endohedral position, the larger atoms form exohedral capping of the host cluster. The stability of these clusters has been analyzed based on the average binding energy, interaction energy of the impurity atoms, and the energy gap between the highest occupied and lowest unoccupied energy levels (HLG). Based on the energetics, it is found that p-p interaction dominates over the s-p interaction and smaller size atoms interact more strongly. The stability analysis of these clusters suggests that, while the substitution of Pb by C or Al enhances the stability of the Pbn clusters, Mg lowers the stability. Further investigations of the stability of PbnM clusters reveal that the interplay between the atomic and electronic structure is crucial to understand the stability of these clusters. The energy gap analysis reveals that, while the substitution of Mg atom widens the HLG, all other elements reduce the gap of the PbnM clusters.

We report the structural and electronic properties of Aun (n = 1-7 and 10) clusters supported on a clean α-Al2O3(0001) surface using the spin-polarized version of the plane wave based pseudo-potential method. To underscore the effect of support interaction, the geometries of the deposited Au clusters are compared with the gas-phase structures. In general, the trend in the growth pattern shows that all deposited Au clusters favor planar configurations, similar to that in the isolated case. However, due to the roughness of the Al2O3(0001) surface the deposited Au atoms are arranged in a zig-zag pattern. The binding energy of an Au atom on the Al2O3 surface is 0.79 eV and it binds to the surface Al atom. Two Au atoms prefer to form a dimer on the alumina surface rather than adsorbing as a monomer at a long distance. As the size of the cluster increases the adsorption energy shows a decreasing trend. The nature of chemical bonding at the interface is established by the charge distribution analysis, which suggests an overall charge transfer from the surface to the Au cluster. The additional negative charge on the deposited Au cluster corroborates the red shift of the energy levels of the Au-Al2O3 composite in comparison to the isolated Aun clusters. Further investigations were carried out by analyzing the interaction between the oxygen molecule and the Aun@Al2O3 system, a prototype to study the oxidation mechanism. The results reveal that the interaction of O2 with Aun@Al2O3 follows a dissociative chemisorption route and ruptures the O-O bond.