Assigning effective atomic charges that properly reproduce the electrostatic fields of molecules is a crucial step in the construction of accurate interatomic potentials. We propose a new approach to calculate these charges, which as previous approaches are, is based on the idea of charge equilibration. However, we only allow charge to flow between covalently bonded neighbors by using the concept of so-called split charges. The semiempirical fit parameters in our approach do not only reflect atomic properties (electronegativity and atomic hardness) but also bond-dependent properties. The new method contains two popular but hitherto disjunct approaches as limiting cases. We apply our methodology to a set of molecules containing the elements silicon, carbon, oxygen, and hydrogen. Effective charges derived from electrostatic potential surfaces can be predicted more than twice as accurately as with previous works, at the expense of one additional fit parameter per bond type controlling the polarizability between two bonded atoms. Additional bond-type parameters can be introduced, but barely improve the results. An increase in accuracy of only 30% over existing techniques is achieved when predicting Mulliken charges. However, this could be improved with additional bond-type parameters.

A triple basis set in Hylleraas coordinates is used to obtain improved variational bounds for the nonrelativistic energy and other properties of He, H Ϫ , and Ps Ϫ. The accuracy, numerical stability, and computational efficiency are compared with recent work based on quasirandom basis sets. The Kato cusp conditions are used to assess the accuracy of the wave functions at short distances.

Graphene forms an important two-dimensional (2D) material class that displays both a high electronic conductivity and optical transparency when doped. Yet, the microscopic origin of the doping mechanism in single sheet or bulk intercalated systems remains unclear. Using large-scale ab initio simulations, we show the graphene surface acts as a catalytic reducing/oxidizing agent, driving the chemical disproportionation of adsorbed dopant layers into charge-transfer complexes which inject majority carriers into the 2D carbon lattice. As pertinent examples, we focus on the molecular SbCl(5) and HNO(3) intercalates, and the solid compound AlCl(3). Identifying the microscopic mechanism for the catalytic action of graphene is important, given the availability of large area graphene sheets, to spur research into new redox reactions for use in science and technology.

We investigate the generic dielectric properties of solids in the split-charge equilibration ͑SQE͒ formalism, which contains the regular charge equilibration ͑QE͒ method as a limiting case, but augments it with a bond hardness term. It is shown that QE always mimics ideal conductors, while any positive bond hardness used in SQE turns the solid into a dielectric. Crystals with simple cubic and rocksalt structure are considered explicitly. For these symmetries, we solve the continuum limit of the SQE formalism analytically. As a result, we provide simple analytical expressions for how dielectric constant and penetration depth of the electrostatic field depend on atomic hardness, bond hardness, and lattice constant. This mapping may prove useful not only for force field parametrization but also for solving dielectric responses on coarse-grained scales. Successful comparison of numerical data to analytical solutions is made, including those containing discretization corrections.

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