Nonlocal Excitations and 1/8 Singularity in Cuprates

Momentum-dependent excitation spectra of the two-dimensional Hubbard model on the square lattice have been investigated at zero temperature on the basis of the full self-consistent projection operator method in order to clarify nonlocal effects of electron correlations on the spectra. It is found that intersite antiferromagnetic correlations cause shadow bands and enhance the Mott-Hubbard splittings near the half-filling. Furthermore nonlocal excitations are shown to move the critical doping concentration $\delta^{\ast}_{h}$, at which the singular quasiparticle peak is located just on the Fermi level, from $\delta^{\ast}_{h}=0.153$ (the single-site value) to $\delta^{\ast}_{h}=0.123$. The latter suggests the occurance of an instability such as the stripe at $\delta^{\ast}_{h}=1/8$.Comment: 4 pages, 5 figures; to be published in the Journal of Korean Physical Society (ICM12

Correlation effects in MgO and CaO: Cohesive energies and lattice constants

A recently proposed computational scheme based on local increments has been applied to the calculation of correlation contributions to the cohesive energy of the CaO crystal. Using ab-initio quantum chemical methods for evaluating individual increments, we obtain 80% of the difference between the experimental and Hartree-Fock cohesive energies. Lattice constants corrected for correlation effects deviate by less than 1% from experimental values, in the case of MgO and CaO.Comment: LaTeX, 4 figure

Halogen Bond Asymmetry in Solution

Halogen bonding is the noncovalent interaction of halogen atoms in which they act as electron acceptors. Whereas three-center hydrogen bond complexes, [D center dot center dot center dot H center dot center dot center dot D](+) where D is an electron donor, exist in solution as rapidly equilibrating asymmetric species, the analogous halogen bonds, [D center dot center dot center dot X center dot center dot center dot D](+), have been observed so far only to adopt static and symmetric geometries. Herein, we investigate whether halogen bond asymmetry, i.e., a [D-X center dot center dot center dot D](+) bond geometry, in which one of the D-X bonds is shorter and stronger, could be induced by modulation of electronic or steric factors. We have also attempted to convert a static three-center halogen bond complex into a mixture of rapidly exchanging asymmetric isomers, [D center dot center dot center dot X-D](+) (sic) [D-X center dot center dot center dot D](+), corresponding to the preferred form of the analogous hydrogen bonded complexes. Using N-15 NMR, IPE NMR, and DFT, we prove that a static, asymmetric geometry, [D-X center dot center dot center dot D](+), is obtained upon desymmetrization of the electron density of a complex. We demonstrate computationally that conversion into a dynamic mixture of asymmetric geometries, [D center dot center dot center dot X-D](+) (sic) [D-X center dot center dot center dot D](+), is achievable upon increasing the donor-donor distance. However, due to the high energetic gain upon formation of the three-center-four electron halogen bond, the assessed complex strongly prefers to form a dimer with two static and symmetric three-center halogen bonds over a dynamic and asymmetric halogen bonded form. Our observations indicate a vastly different preference in the secondary bonding of H+ and X+. Understanding the consequences of electronic and steric influences on the strength and geometry of the three-center halogen bond provides useful knowledge on chemical bonding and for the development of improved halonium transfer agents