Jason K. Perry

Antiferromagnetic band structure ofLa2CuO4: Becke-3–Lee-Yang-Parr calculations

Using the Becke-3-LYP functional, we have performed band structure calculations on the high temperature superconductor parent compound, La2CuO4. Under the restricted spin formalism (ρ↑ = ρ↓), the R-B3LYP band structure agrees well with the standard LDA band structure. It is metallic with a single Cu x − y/O pσ band crossing the Fermi level. Under the unrestricted spin formalism (ρ↑ 6= ρ↓), the U-B3LYP band structure has a spin polarized antiferromagnetic solution with a band gap of 2.0 eV, agreeing well with experiment. This state is 1.0 eV (per formula unit) lower than that calculated from the R-B3LYP. The apparent high energy of the spin restricted state is attributed to an overestimate of on-site Coulomb repulsion which is corrected in the unrestricted spin calculations. The stabilization of the total energy with spin polarization arises primarily from the stabilization of the x − y band, such that the character of the eigenstates at the top of the valence band in the antiferromagnetic state becomes a strong mixture of Cu x − y/O pσ and Cu z /O pz. Since the Hohenberg-Kohn theorem requires the spin restricted and spin unrestricted calculations to give identical ground state energies and total spatial densities for the exact functionals, this large disparity in energy reflects the inadequacy of current functionals for describing the cuprates. This calls into question the use of band structures based on current restricted spin density functionals (including LDA) as a basis for single band theories of superconductivity in these materials.

Theoretical studies of a hydrogen abstraction tool for nanotechnology

In the design of a nanoscale, site-specific hydrogen abstraction tool, the authors suggest the use of an alkynyl radical tip. Using ab initio quantum-chemistry techniques including electron correlation they model the abstraction of hydrogen from dihydrogen, methane, acetylene, benzene and isobutane by the acetylene radical. By conservative estimates, the abstraction barrier is small (less than 7.7 kcal mol^-1) in all cases except for acetylene and zero in the case of isobutane. Thermal vibrations at room temperature should be sufficient to supply the small activation energy. Several methods of creating the radical in a controlled vacuum setting should be feasible. The authors show how nanofabrication processes can be accurately and inexpensively designed in a computational framework.

Ab initio evidence for the formation of impurity d3z2-r2 holes in doped La2-xSrxCuO4

Using the spin unrestricted Becke-3-Lee-Yang-Parr density functional, we computed the electronic structure of explicitly doped La2-xSrxCuO4 (x=0.125, 0.25, and 0.5). At each doping level, an impurity hole band is formed within the undoped insulating gap. This band is well localized to CuO6 octahedra adjacent to the Sr impurities. The nature of the impurity hole is A1g in symmetry, formed primarily from the z2 orbital on the Cu and pz orbitals on the apical O’s. There is a strong triplet coupling of this hole with the intrinsic B1g Cu x2-y2/O1 pσ hole on the same site. Optimization of the c coordinate of the apical O’s in the doped CuO6 octahedron leads to an asymmetric anti-Jahn-Teller distortion of the O_2 atoms toward the central Cu. In particular, the O_2 atom between the Cu and Sr is displaced 0.26 A while the O_2 atom between the Cu and La is displaced 0.10 A. Contrary to expectations, investigation of a 0.1 A enhanced Jahn-Teller distortion of this octahedron does not force formation of an x^2 - y^2 hole, but instead leads to migration of the z^2 hole to the four other CuO_6 octahedra surrounding the Sr impurity. This latter observation offers a simple explanation for the bifurcation of the Sr-O_2 distance revealed in x-ray absorption fine structure data.

Inequivalence of equivalent bonds: Symmetry breaking in Co(CH3)2+

In a theoretical study of the gas phase insertion of transition‐metal cations into the C–H and C–C bonds of simple alkanes, an unusual aspect of the metal‐carbon bond was discovered. Using ab initio methods (generalized valence bond and configuration interaction), it was found that the two methyl groups in Co(CH3)2+ do not bond to equivalent sd hybrid orbitals as one might expect. Instead, using a single valence bond (VB) spin coupling, we found two distinctly different bonds: one to a Co 4s‐like orbital and the other to a 3d‐like orbital, leading to a distortion of the molecule from its symmetrical geometry. With the resonance of two valence bond configurations, the bond distances become equivalent and symmetry is restored, however, the bonding orbitals in each configuration remain quite inequivalent. Similar behavior was observed on the potential‐energy surface of CoH2+ and this description was found to carry over to Co(H)(CH3)+, where one VB configuration dominates: the hydrogen bonds to the Co 4s orbi...