First principles investigations of Fe,Co, Ni in model honeycomb carbon networks

The behaviors of ferromagnetic transition metals of the first period: Fe, Co and Ni are examined within density functional theory calculations in two dimensional carbon extended networks using model structure LiC₆. Around geometry optimized structures, the energy-volume equations of states considering non magnetic and spin polarized configurations established ferromagnetic ground states with magnetizations –reduced with respect to the metals’– of 2 μ_B for FeC₆ and 1 μ_B for CoC₆ while no magnetic solution could be identified for NiC₆. In the D_6h point group of the P6/mmm space group l_m decomposition of the d states results with increasing energy into doublet state E_1g with d(x²-y²) and d(xy); singlet state A_1g d(z²) and doublet state E_2g d(xz) and d(yz) lying on E_F and responsible of the onset of magnetic moments. This was mirrored via molecular orbital approach with a construct of Fe embedded between two extended carbon networks thus validating the model structure proposed for TC₆ compounds. The 100% polarization in one spin channel allows proposing potential uses in spintronics applications.     

Numerical examination of performance of some exchange-correlation functionals for molecules containing heavy elements

The performance of 17 exchange-correlation functionals for molecules containing heavy elements are numerically examined through four-component relativistic density DFT calculations. The examined functionals show the similar accuracy as they do for the molecules containing light elements only except for bond lengths. LDA and OP86 produce good results for bond lengths and frequencies but bad bond energies. Different functionals do not show much different performance for bond energies except LDA. BP86 and GP86 produce results with average accuracy while LYP does not perform well. Although encouraging results are obtained with functional B97GGA-1, other heavily parameterized and meta-GGA functionals do not produce impressive results.     

Prediction of semiconducting behavior in minority spin of Co2CrZ（Z=Ga,Ge,As）:LSDA

正Volume optimization was performed to obtain the theoretical lattice constants by using the generalized gradient approximation(GGA).The electronic and magnetic properties of Heusler alloys Co_2CrZ(Z = Ga,Ge, As) were investigated by using local spin density approximation(LSDA).Amongst the systems under investigation, Co_2CrGe and Co_2CrGa give 100%spin polarization at the Fermi level(E_f)- Co_2CrGe and Co_2CrGa are the most stable half-metallic ferromagnets(HMFs);their E_F lie exactly at the gap of 0.24 eV and 0.38 eV,respectively,in the spin-down channel.Even though Co_2CrAs gives a distinct and bigger gap as compared to Co_2CrGa and Co_2CrGe, its E_F is not located at the middle of the gap in the spin-down channel.We have also found that the total magnetic moments increase as the Z goes from Ga to As.The calculated density of states and band structures show the HMF character for Co_2CrGe and Co_2CrGa.     

Electronic and optical properties of high pressure stable phases of ZnS: Comparison of FPLAPW and PW-PP results

A theoretical study of the structural phase transformation of ZnS under high pressure has been performed using first principle plane wave pseudopotential (PW-PP) and full potential linear augmented plane wave method (FPLAPW) calculation in which Zn-3d states are treated as valence states. In both methods, we have used a generalized gradiant approximation for the study of phase transformation and structural parameters. The calculated difference in lattice constants (Δα₀) by PW-PP and FPLAPW methods for zinc-blende, cinnabar and rocksalt structures is equal to 0.003, 0.01 and 0.001 Å respectively. There is a very good agreement between the results of PW-PP and FPLAPW calculations that shows soundness of our choice of pseudopotential. The calculated transition pressure for zinc-blende → rocksalt is in agreement with available measured data. We present calculations of the optical properties for three phases of ZnS. The band gap of different phases of ZnS decreases in order of zinc-blende → cinnabar → rocksalt mainly due to red shift of Zn-s states in the lowest conduction band. Besides, the optical band gap decreases from 2.84 eV (direct) to 0.188 eV (indirect). The shift of calculated complex dielectric function ε₂(ω) for zinc-blende → cinnabar → rocksalt is also discussed in details of optical transition that occurred in different phases.     

Investigation of the (√3 × √3)R30°-Cu2Si/Cu(1 1 1) surface alloy using DFT

The electronic structure of the FCC, HCP and 2-fold bridge phases of the (√3 × √3)R30°-Cu₂Si/Cu(1 1 1) surface alloy have been investigated using LCAO-DFT. Analysis of the total electron density, partial density-of-states (PDOS) and crystal orbital overlap population (COOP) curves for the system have shown a surprising similarity between the intra- and inter-layer Si-Cu bond for each phase. Low hybridization between the Si 3s and 3p orbitals results in a low directionality of the Si-Cu bond within each of phase. The Si 3s orbitals are shown to form covalent bonds with their surrounding Cu atoms whereas the Si 3p and 3d orbitals are shown to form combinations of covalent and metallic bonds. The Si-Cu interaction is shown clearly to extend to the second layer of the alloy in deference to previous studies of Si/Cu alloys.     

A sensitive, rapid ferricyanide-mediated toxicity bioassay developed using Escherichia coli

A need for rapid toxicity techniques has seen recent research into developing new microbiological assays and characterising their toxicity responses using a range of substances. A microbiological bioassay that determines changes in ferricyanide-mediated respiration for toxicity measurement (FM-TOX) shows particular promise. The development and optimisation of an improved FM-TOX method, incorporating novel features, is described using Escherichia coli as the biocatalyst. Omission of an exogenous carbon source, used in previously described FM-TOX assays, substantially improves the assay sensitivity. In addition, the development of a two-step procedure (toxicant exposure followed by determination of microbial respiratory activity) was found to enhance the inhibition of E. coli by 3,5-dichlorophenol and four other toxicants, compared to single-step procedures. Other assay parameters, such as the ferricyanide concentration, exposure times and optical density of the biocatalyst were also optimised, sometimes based on practical aspects. Toxicity tests were carried out using the adopted technique on both organic and inorganic toxicants, with classic sigmoid-shaped dose-response curves observed, as well as some non-standard responses. IC₅₀ data is presented for a number of common toxicants. The optimised assay provides a good foundation for further toxicity testing using E. coli, as well as the potential for expanding the technique to utilise other bacteria with complementary toxicity responses, thereby allowing use of the assay in a range of applications.     

含B闪锌矿三元系半导体BxIn1-xP和BxGa1-xP带隙理论预测

利用第一性原理计算方法,用广义梯度近似 (GGA) 处理电子之间的交换关联能,计算了BxIn1-xP,BxGa1-xP的带隙结构.BxIn1-xP的带隙弯曲参数b _Eg (Γ)=4.32eV,b_Eg(X) =1.8eV,它从直接带隙转变为间接带隙时B的含量为0.47;在小量B的掺入下,可使BxIn1-xP的带隙变小,在x=0.15时带隙达到最小值1.33eV.BxGa1-xP的带隙弯曲参数b_Eg(Γ)=1.37eV,b _Eg(X)=2.46eV,在整个组分内是间接带隙.BxIn1-x-P,BxGa1-xP有较大的带隙弯曲参数,是由于组成它们的二元系之间存在较大的晶格失配.     

Is combining meta‐GGA correlation functionals with the OPTX exchange functional useful?

The recent generalized gradient approximation (GGA) density functional OCS1 of Handy and Cohen is implemented in the deMon code and tested on a carefully selected set of problems. OCS1 is found to be accurate for molecular atomization energies, transition metal–ligand bonds, and systems with intramolecular hydrogen bonds. However, OCS1 encounters problems for systems with intermolecular hydrogen bonds. It also tends to elongate bond lengths systematically, and sometimes significantly. The OPTX exchange is combined with three meta‐GGA correlation functionals, Lap3, τ1, and τ2, the latter reported for the first time. The new meta‐GGA scheme OPTX exchange plus τ2 correlation called Oτ2 yields improved molecular geometries, NMR shielding constants, and an improved barrier height for the H+H₂ reaction. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006