Computational Discovery of A New Rhombohedral Diamond Phase

We identify by first-principles calculations a new diamond phase in R¯3c (D63d) symmetry, which has a 16-atom rhombohedral primitive cell, thus termed R16 carbon. This rhombohedral diamond comprises a characteristic all-sp3 six-membered-ring bonding network, and it is energetically more stable than previously identified diamondlike six-membered-ring bonded BC8 and BC12 carbon phases. A phonon mode analysis verifies the dynamic structural stability of R16 carbon, and electronic band calculations reveal that it is an insulator with a direct band gap of 4.45 eV. Simulated x-ray diffraction patterns provide an excellent match to recently reported distinct diffraction peaks found in milled fullerene soot, suggesting a viable experimental synthesis route. These findings pave the way for further exploration of this new diamond phase and its outstanding properties

ab initio Study of Strain-Induced Ferroelectricity in SrTiO3

Valley lines on total-energy surfaces for the zone-center distortions of free-standing and in-plane strained SrTiO3 are investigated with a newly developed first-principles structure optimization technique [Jpn. J. Appl. Phys. vol.43 (2004), p.6785]. The results of numerical calculations confirmed that the ferroelectricity is induced, and the Curie temperature is increased, by applying biaxial compressive or tensile strains. Along the distortion, strong nonlinear coupling between the soft- and hard-modes is demonstrated.Comment: 15 pages, 10 figures, submitted to Jpn. J. Appl. Phy

Ab initio Study of Valley Line on a Total-Energy Surface for Zone-Center Distortions of Ferroelectric Perovskite Oxides BaTiO3 and PbTiO3

An ab initio structure optimization technique is newly developed to determine the valley line on a total-energy surface for zone-center distortions of ferroelectric perovskite oxides and is applied to barium titanate BaTiO3 and lead titanate PbTiO3. The proposed technique is an improvement over King-Smith and Vanderbilt's scheme [Phys. Rev. B 49, p.5828 (1994)] of evaluating total energy as a function of the amplitude of atomic displacements. The results of numerical calculations show that total energy can be expressed as a fourth-order function of the amplitude of atomic displacements in BaTiO3 but not in PbTiO3.Comment: 4 pages, 5 figure

Probing the Structure, Stability and Hydrogen Adsorption of Lithium Functionalized Isoreticular MOF-5 (Fe, Cu, Co, Ni and Zn) by Density Functional Theory

Li adsorption on isoreticular MOFs with metal Fe, Cu, Co, Ni and Zn was studied using density function theory. Li functionalization shows a considerable structural change associated with a volume change in isoreticular MOF-5 except for the Zn metal center. Hydrogen binding energies on Li functionalized MOFs are seen to be in the range of 0.2 eV, which is the desired value for an ideal reversible storage system. This study has clearly shown that Li doping is possible only in Zn-based MOF-5, which would be better candidate to reversibly store hydrogen

Influence of molecular geometry, exchange-correlation functional, and solvent effects in the modeling of vertical excitation energies in phthalocyanines using time-dependent density functional theory (TDDFT) and polarized continuum model TDDFT methods: can modern computational chemistry methods explain experimental controversies?

A time-dependent density functional theory (TDDFT) approach coupled with 14 different exchange-correlation functionals was used for the prediction of vertical excitation energies in zinc phthalocyanine (PcZn). In general, the TDDFT approach provides a more accurate description of both visible and ultraviolet regions of the UV-vis and magnetic circular dichroism (MCD) spectra of PcZn in comparison to the more popular semiempirical ZINDO/S and PM3 methods. It was found that the calculated vertical excitation energies of PcZn correlate with the amount of Hartree-Fock exchange involved in the exchange-correlation functional. The correlation was explained on the basis of the calculated difference in energy between occupied and unoccupied molecular orbitals. The influence of PcZn geometry, optimized using different exchange-correlation functionals, on the calculated vertical excitation energies in PcZn was found to be relatively small. The influence of solvents on the calculated vertical excitation energies in PcZn was considered for the first time using a polarized continuum model TDDFT (PCM-TDDFT) method and was found to be relatively small in excellent agreement with the experimental data. For all tested TDDFT and PCM-TDDFT cases, an assignment of the Q-band as an almost pure a_(1u) (HOMO)-->e_g (LUMO) transition, initially suggested by Gouterman, was confirmed. Pure exchange-correlation functionals indicate the presence of six ^1_Eu states in the B-band region of the UV-vis spectrum of PcZn, while hybrid exchange-correlation functionals predict only five ^1E_u states for the same energy envelope. The first two symmetry-forbidden n-->pi* transitions were predicted in the Q0-2 region and in the low-energy tail of the B-band, while the first two symmetry-allowed n-->π* transitions were found within the B-band energy envelope when pure exchange-correlation functionals were used for TDDFT calculations. The presence of a symmetry-forbidden but vibronically allowed n-->π* transition in the Q_(0-2) spectral envelope explains the long-time controversy between the experimentally observed low-intensity transition in the Q_(0-2) region and previous semiempirical and TDDFT calculations, which were unable to predict any electronic transitions in this area. To prove the conceptual possibility of the presence of several degenerate ^1E_u states in the B-band region of PcZn, room-temperature UV-vis and MCD spectra of zinc tetra-tert-butylphthalocyanine (Pc^tZn) in non-coordinating solvents were recorded and analyzed using band deconvolution analysis. It was found that the B-band region of the UV-vis and MCD spectra of Pc^tZn can be easily deconvoluted using six MCD Faraday A-terms and two MCD Faraday B-terms with energies close to those predicted by TDDFT calculations for ^1E_u and ^1A_(2u) excited states, respectively. Such a good agreement between theory and experiment clearly indicates the possibility of employing a TDDFT approach for the accurate prediction of vertical excitation energies in phthalocyanines within a large energy range

Energetics and local spin magnetic moment of single 3,4d impurities encapsulated in an icosahedral Au12 cage

The energetics and local spin magnetic moment of a single 3,4d impurity (Sc-Ni, Y-Pd) encapsulated in an icosahedral Au12 cage have been studied theoretically by using a real-space first-principles cluster method with generalized gradient approximation for exchange-correlation functional. The relativistic effect is considered by scalar relativistic pseudopotentials. All doped clusters show unexpected large relative binding energies compared with icosahedral Au13cluster. The smallest and the largest values appear at Pd and Zr, 2.186 and 7.791eV per cluster, respectively, indicating doping could stabilize the icosahedral Au12 cage and promote the formation of a new binary alloy cluster. Comparatively large magnetic moments are observed for 3d elements Cr, Mn, Fe, Co, and Ni (2.265, 3.512, 3.064, 1.947, and 0.943μB), and 4d elements Tc, Ru, and Rh (0.758, 1.137, and 0.893μB). The density of states and the relativistic effects on electronic structure are discussed