Within the framework of ab initio simulation, a number of modifications of well-known carbon nanostructures are proposed, which could form the basis for designing materials with high adsorptivity for molecular hydrogen.

On the basis of first-principles calculations of molecular electron structure, we discuss the strategy of modifying the carbon-based materials in order to increase their capacity to bind with molecular hydrogen. In particular, we have studied hydrogen adsorption on molecular complexes having anionic aromatic carbon-based rings stabilized by cations of alkali (Li + , Na + , K + ) or alkali-earth metals (Be 2+ , Mg 2+ , Ca 2+ ). The adsorption depends more on the properties of the cation than on the ring itself. The interaction of the H 2 molecule with an electrostatic field leads to the binding of the hydrogen molecule with the strongly polarized ionic molecular complex. The number of the adsorbed molecules is driven by two factors acting in opposite directions: the binding energy, which should be larger than a 4−5 kJ/mol threshold needed to keep hydrogen molecules attached, and the area around the cation (coordination sphere), which is determined by its radius. As a compromise between these factors, we propose several promising candidates for building blocks of hydrogen storage materials, including diboratabenzene lithium, C 4 B 2 H 6 Li 2 , and diboratabenzene potassium, C 4 B 2 H 6 K 2 , which can adsorb 6 and 12 H 2 molecules, correspondingly. We also discuss the possibility of linking these molecular complexes in three-dimensional structures.

Within the resolution of the identity (RI) method, the convergence of the Hartree-Fock (HF) total molecular energy and the multipole moments in the course of the combined regular expansion of the molecular and auxiliary (RI) basis sets is studied. Dunning's cc-pVXZ series is used for both the molecular and the RI basis sets. The results show the calculated quantities converge to the HF limit when both the molecular and the RI basis sets are expanded from correlation-consistent polarized valence double zeta to correlation-consistent polarized valence sextuple zeta. Combinations of molecular/RI basis sets sufficient for convergence of the total energy and of the multipole moments at various accuracy levels have been determined. A measure of the RI basis set incompleteness is suggested and discussed. As it is significantly faster than the standard HF algorithm for small and midsize molecules, the RI-HF method, together with appropriate expanding series of both molecular and RI basis sets, provide an efficient tool to estimate and control the error of the Hartree-Fock calculations due to the finite basis set.

We study one-dimensional ͑1D͒ carbon ribbons with the armchair edges and the zigzag carbon nanotubes and their counterparts with finite length ͓zero dimension ͑0D͔͒ in the framework of the Hückel model. Using boundary conditions we derive energy spectra for 1D carbon ribbons. At the Fermi level we construct the explicit solutions and prove the rule of metallicity. We show that the dispersion law ͑electron band energy͒ of a 1D metallic ribbon or a 1D metallic carbon nanotube has a universal sinelike dependence at the Fermi energy which is independent of its width. We find that in case of metallic graphene ribbons of finite length ͑rectangular graphene macromolecules͒ or nanotubes of finite length the discrete energy spectrum in the vicinity of =0 ͑Fermi energy͒ can be obtained exactly by selecting levels from the same dispersion law. In case of a semiconducting graphene macromolecule or a semiconducting nanotube of finite length, the positions of energy levels around the energy gap can be approximated with a good accuracy. The electron spectrum of 0D carbon structures often includes additional states at energy = 0, which are localized on zigzag edges and do not contribute to the volume conductivity.

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