Topology, connectivity and electronic structure of C and B cages and the corresponding nanotubes

After a brief discussion of the structural trends which appear with increasing number of atoms in B cages, a one-to one correspondence between the connectivity of B cages and C cage structures will be proposed. The electronic level spectra of both systems from Hartree-Fock calculations is given and discussed. The relation of curvature introduced into an originally planar graphitic fragment to pentagonal 'defects' such as are present in buckminsterfullerene is also briefly treated. A study of the structure and electronic properties of B nanotubes will then be introduced. We start by presenting a solution of the free-electron network approach for a 'model boron' planar lattice with local coordination number 6. In particular the dispersion relation E(k) for the pi-electron bands, together with the corresponding electronic Density Of States (DOS), will be exhibited. This is then used within the zone folding scheme to obtain information about the electronic DOS of different nanotubes obtained by folding this model boron sheet. To obtain the self-consistent potential in which the valence electrons move in a nanotube, 'the March model' in its original form was invoked and results are reported for a carbon nanotube. Finally, heterostructures, such as BN cages and fluorinated buckminsterfullerene, will be briefly treated, the new feature here being electronegativity difference.Comment: 22 pages (revtex4) 12 figure

Intermolecular Pauli repulsion: a QMC study of molecules in ground and excited state in free space and in solution

In this work we present a method to compute the Pauli repulsion interaction energy between two molecules and for a molecule solvated by a discrete medium. The method of Amovilli and Mennucci, that has been developed within a continuum solvent model approach, is here revised in order to treat the solvation environment with a discrete number of solvent molecules. In our model, one of the two interacting systems, and the solvent in the case of solvation, acts as ‘probe’. A probe has a volume domain defined by the atomic spheres centred on the nuclei of the relevant molecule. The probe measures the fraction of electrons of the solute molecule falling in its domain leading to the evaluation of Pauli repulsion energy. To this end, Quantum Monte Carlo calculations are used to sample the electronic configurations of the solute. The method has been designed to be applied also to excited states. We show results for test systems in the ground state and for the ground and the (Formula presented.) excited states of acetone in a cluster with 14 water molecules

Shannon Entropy in Atoms: A Test for the Assessment of Density Functionals in Kohn-Sham Theory

Electron density is used to compute Shannon entropy. The deviation from the Hartree-Fock (HF) of this quantity has been observed to be related to correlation energy. Thus, Shannon entropy is here proposed as a valid quantity to assess the quality of an energy density functional developed within Kohn-Sham theory. To this purpose, results from eight different functionals, representative of Jacob's ladder, are compared with accurate results obtained from diffusion quantum Monte Carlo (DMC) computations. For three series of atomic ions, our results show that the revTPSS and the PBE0 functionals are the best, whereas those based on local density approximation give the largest discrepancy from DMC Shannon entropy