O与O2在Au团簇上吸附的密度泛函理论研究

采用基于密度泛函理论(DFT)的Dmol³程序系统研究了O原子与O₂在 Au₁₉与Au₂₀团簇上的吸附反应行为. 结果表明: O在Au₁₉团簇顶端洞位上的吸附较Au₂₀强; 在侧桥位吸附强度相近. O与O₂在带负电Au团簇上吸附较强, 在正电团簇吸附较弱. 从O―O键长看, 当金团簇带负电时, O―O键长较长, 中性团簇次之, 正电团簇中O―O键长较短, 因而O₂活化程度依次减弱. 电荷布居分析表明, Au团簇带负电时, O与O₂得电子数较中性团簇多, 而团簇带正电时, 得电子数较少. 差分电荷密度(CDD)表明, O₂与Au团簇作用时, 金团簇失电子, O₂的π^*轨道得电子, 使O―O键活化. O₂在Au₁₉^-团簇上解离反应活化能为1.33 eV, 较中性团簇低0.53 eV; 而在Au₁₉⁺上活化能为2.27 eV, 较中性团簇高0.41 eV, 这与O₂在不同电性Au₁₉团簇O―O键活化规律相一致.     

BaTiO3的电子结构和光学性质

采用第一性原理赝势平面波方法, 在局域密度近似(LDA)和广义梯度近似(GGA)下分别计算了BaTiO3立方相和四方相的电子结构, 并在局域密度近似下计算了BaTiO3立方相的光学性质. 结果表明, BaTiO3立方相和四方相都为间接带隙, 方向分别为Γ-M和Γ-X, 大小分别为2.02和2.20 eV. 对BaTiO3和PbTiO3铁电相短键上电子布居数的对比分析, 给出了它们铁电性大小的差别. 且在30 eV的能量范围内研究了BaTiO3 的介电函数、吸收系数、折射系数、湮灭系数、反射系数和能量损失系数等光学性质,并基于电子能带结构对光学性质进行了解释. 计算结果与实验数据相符合.     

甲硫醇在Cu(111)表面的解离吸附: 密度泛函理论研究

采用基于密度泛函理论的第一性原理方法和平板模型研究了CH₃SH分子在Cu(111)表面的吸附反应.系统地计算了S原子在不同位置以不同方式吸附的一系列构型, 第一次得到未解离的CH₃SH分子在Cu(111)表面顶位上的稳定吸附构型,该构型吸附属于弱的化学吸附, 吸附能为0.39 eV. 计算同时发现在热力学上解离结构比未解离结构更加稳定. 解离的CH₃S吸附在桥位和中空位之间, 吸附能为0.75-0.77 eV. 计算分析了未解离吸附到解离吸附的两条反应路径, 最小能量路径的能垒为0.57 eV. 计算结果还表明S―H键断裂后的H原子并不是以H₂分子的形式从表面解吸附而是以与表面成键的形式存在. 通过比较S原子在独立的CH₃SH分子和吸附状态下的局域态密度, 发现S―H键断裂后S原子和表面的键合强于未断裂时S原子和表面的键合.     

取代基对咔咯锰(V)-氧配合物Mn―O的成键影响

用密度泛函理论(DFT)方法对一系列A₃型咔咯锰(V)氧配合物进行了理论计算. 结果表明: 咔咯锰(V)-氧配合物中Mn―O键是由1 个σ键和2 个π键构成的叁键结构; 当咔咯中位取代基由推电子过渡到拉电子性质时, 咔咯骨架紧缩, Mn―O键缩短, 其拉曼光谱的伸缩振动峰往高波数移动; 取代基与氧原子的静电作用模式由正-负吸引转化为负-负排斥, 导致Mn―O键解离能ΔE减少, 即拉电子基团有利于增强咔咯锰(V)-氧配合物氧原子的活泼性.     

六角多铁性HoMnO3的电子和能带结构

基于密度泛函理论(DFT)结合投影缀加平面波(PAW)方法, 运用广义梯度近似(GGA), 在考虑电子基态自旋阻挫非共线的磁性结构基础上, 研究了具有六角钙钛矿结构HoMnO₃材料的磁性、电子和能带结构, 并解释了相关实验结果. 结果表明: 当考虑Mn³⁺离子的电子自旋在平面内呈阻挫的三角非共线反铁磁(NAFM)排列时, 六角HoMnO₃的总能降低、能隙变大、磁矩增大、各原子的位置更接近于实验值, 电子态密度(DOS)分布具有与X光吸收谱测量更为一致的结果. 对非共线磁性结构计算得到的电子态密度和能带结构的分析发现, 实验中观察到的1.7和2.3 eV两个光学吸收峰都源于Mn³⁺离子3d与平面内O(3, 4) 2p形成的杂化态与Mn [3d_3z²-r²]之间的电子跃迁, 而Ho 5d空轨道与平面上的O(3, 4) 2p轨道之间在z方向的强烈杂化驱动HoMnO₃产生垂直于平面方向的铁电极化.     

密度泛函理论研究十二烷硫醇在Au(111)面上的吸附

采用第一性原理方法研究了十二烷硫醇(C₁₂H₂₅SH)分子在Au(111)面上未解离和解离吸附的结构、能量和吸附性质,在此基础上分析判断长链硫醇分子在Au(111)面吸附时S―H键的解离, 以及分子链长度对吸附结构和能量的影响. 计算了S原子在不同位置以不同方式吸附的系列构型, 结果表明在S―H键解离前和解离后,均存在两种可能的表面结构, 直立吸附构型和平铺吸附构型; 未解离的C₁₂H₂₅SH分子倾向于吸附在top位, 吸附能为0.35-0.38 eV; H原子解离后C₁₂H₂₅S基团倾向于吸附在bri-fcc位, 吸附能量为2.01-2.09 eV. 比较分析未解离吸附和解离吸附, 发现C₁₂H₂₅SH分子未解离吸附相较于解离吸附要稳定, 未解离吸附属于弱化学吸附.局域电子态密度和差分电荷密度分析进一步验证了S―H解离后S原子与表面之间成键的数目增加, 而且键合更强. 同时我们发现长链硫醇的吸附能量较短链硫醇的吸附能量略大, S原子与表面Au原子之间的距离略小.     

从头计算方法比较研究B2Au4, Al2Au4和BAlAu4的几何和电子结构

Au/H 相似性的研究是现代化学中的一个热门话题. 我们从理论上报道Au/H 相似的新成员: 共价化合物B₂Au₄, 离子化合物Al₂Au₄和BAlAu₄. 采用密度泛函和波函数理论方法对比研究了缺电子体系B₂Au₄、Al₂Au₄和BAlAu₄的几何和电子结构. 详细讨论了它们基态结构的轨道、适应性自然密度划分(AdNDP)和电子局域函数(ELF)分析. 计算结果表明稍微扭曲变形的C₂B₂Au₄是基态结构, 在这个共价化合物中含有两个B―Au―B三中心二电子(3c-2e)键. 然而C_3v Al⁺(AlAu₄)^-和C_3v Al⁺(BAu₄)^-被研究证明是含有三个X―Au―Al 三中心二电子键的类盐化合物(在Al₂Au₄中X=Al, BAlAu₄中X=B). Al₂Au₄和BAlAu₄是至今为止首例报道的在离子缺电子体系中含有金桥键的化合物. 同时计算了B₂Au₄^-、Al₂Au₄^- 和BAlAu₄^- 阴离子基态结构的绝热剥离能和垂直剥离能, 为实验表征提供依据. 文中报道的金桥键为共价键和离子键相结合的缺电子体系提供了一个有趣的键合模式, 有助于设计含有高度分散金原子的新材料和催化剂.     

非血红素铁(III)活化氧分子反应的自旋轨道耦合和零场分裂

采用密度泛函理论对原儿茶酚3,4-双加氧酶(3,4-PCD)活化O₂分子的反应机理进行了探讨. 初始复合物, 六重态⁶1的超快形成主要归因于电子交换诱导系间穿越(EISC), Fe d_z:O₂ π^*(z)是主要的交换通道, 在Fe―O键长为0.2487 nm处, 交换重叠积分S_ij=ád_z α|π^*(z) β>=0.3758. 从六重态⁶1 形成四重态中间体⁴1, 有两种效应共存, 即电子交换耦合作用和自旋轨道耦合(SOC)作用, 且相互竞争. 计算结果表明, 自旋轨道耦合(SOC)作用起主导因素(SOC=353.16 cm^-1). 至于O―O键的解离主要取决于儿茶酚(PCA)最高占据分子轨道(HOMO)的电子转移, 非血红素酶的铁中心仅承担PCA向O₂电子转移的缓冲作用.²²     

锂离子电池LiTi0.25Fe0.75SO4F正极材料的电子结构

采用密度泛函理论平面波赝势的方法,计算了LiFeSO₄F和LiTi_0.25Fe_0.75SO₄F正极材料的电子结构。计算结果表明：当锂嵌入材料后,S、O和F的原子布居变化较小,电子主要填充在过渡金属的3d轨道,导致过渡金属被还原,成为电化学反应的活性中心。在嵌锂态中,锂和氧（氟）之间形成了离子键,而过渡金属（Ti和Fe）与氧（氟）之间则形成了共价键,S-O键的共价性最强。态密度的计算结果则表明：Ti和Fe均保持高自旋排列结构;LiFeSO₄F的两个自旋通道的带隙分别为2.88和2.29 eV,其导电性很差;Ti掺杂使体系的带隙消失,显著地提高了正极材料的导电性;LiTi_0.25Fe_0.75SO₄F系统中Ti-O和Ti-F键均比纯相中的Fe-O和Fe-F键的共价性更强,因此Ti掺杂材料具有更好的结构稳定性。     

钛酸锌的制备及其对高氯酸铵热分解的催化性能

ZnTiO₃ nanocrystals were prepared by sol-gel method, using Zn(NO₃)₂ and Ti(C₄H₉O)₄ in the topic. The as-prepared ZnTiO₃ nanocrystals were characterized by XRD, FTIR and TEM, and the catalytic performance of ZnTiO₃ nanocrystals of different contents for the ammonium perchlorate(AP)decomposition was investigated by thermal analysis. The results indicate that ZnTiO₃ with pure cube structure can be synthesized at 600 ℃ by this procedure,which was spheroid with particle size of about 60~100 nm. The results expressed that the low temperature decomposition peaks of AP is advanced by 17 ℃ and the high temperature decomposition peaks of AP is advanced by 24 ℃ when adding 5% nanoparticle ZnTiO₃ powder. The catalytic effects of ZnTiO₃ powders on the high temperature decomposition of AP are less than that of nanometer metal powders, but all the micron metal powders decrease the low decomposition temperature of AP.     

First-principles local density approximation (generalized gradient approximation) +U study of catalytic CenOm clusters: U value differs from bulk

Ceria possesses strong catalytic properties for CONO(x) removal and H(2) production. Clusters often show more intriguing functionalities than their bulk counterparts. Here, the geometric and electronic structures of Ce(n)O(m) (n=1-4,m=2n-1,2n) clusters are studied for the first time using the projected augmented wave method in density functional theory with detailed assessment of the exchange-correlation functional and the Hubbard parameter U. We note that the U value strongly affects the electronic structures of the oxygen-deficient Ce(n)O(2n-1) clusters, though less so on the stoichiometric Ce(n)O(2n). Furthermore, the local density approximation (LDA)+U method is more accurate than the generalized gradient approximation+U in describing the localization of the 4f electrons of the Ce(n)O(m) clusters. The calculated vibration frequency of the CeO molecule with the LDA+U (U=4 eV) is 818.4 cm(-1), in close agreement with experimental values of 820-825 cm(-1) for the low lying states. Different optimal U values were noted for the ceria cluster (4 eV) and its bulk (6 eV), due to quantum-size and geometric effects. The largely reduced formation energy of an oxygen vacancy indicates that the catalytic effect of the Ce(n)O(m) clusters are far greater than bulk CeO(2).     

Structural, electronic, and optical properties of CaCO3 aragonite

Density functional theory ab initio calculations of the structural parameters, electronic structure, carriers effective masses, and optical absorption of the CaCO₃ aragonite polymorph were performed within the local density and generalized gradient approximations, local density approximation (LDA) and generalized gradient approximation (GGA) respectively. A good agreement between the calculated lattice parameters and experimental results was obtained. Both the LDA and GGA results for CaCO₃ aragonite exhibit very close indirect and direct energy gaps, and the computed effective masses are heavy and anisotropic. Two optical absorption regimes related to distinct electronic transitions are predicted by the calculations.     

一种超硬新材料BeP2N4的电子结构和力学性质及本征硬度

基于密度泛函理论,采用局域密度近似(LDA)和广义梯度近似(GGA)泛函研究了硅铍石、尖晶石结构的 BeP₂N₄ 材料的晶格参数、能带结构、态密度、分态密度、Mulliken布居值和弹性性质, 计算结果与已有的实验值和理论值符合很好. 能带结构和态密度表明两种结构的BeP₂N₄材料是宽的直接带隙的绝缘体材料. 尖晶石结构BeP₂N₄的体弹性模量、剪切模量和弹性模量比硅铍石结构的相应的力学量大得多. 利用Sung等提出的硬度经验判据和Gao等提出的基于Mulliken轨道重叠布居数的共价固体本征硬度计算方法, 预测了两种结构的本征硬度值. 计算结果表明硅铍石结构BeP₂N₄虽然体弹模量小, 但是它并不是一种软的材料, 而是一种易脆的硬度较硬的材料, 随着压力增加硅铍石结构BeP₂N₄的脆性逐渐过渡到延性. 尖晶石结构BeP₂N₄是一种易脆的超硬材料. 采用GGA计算得到的硅铍石BeP₂N₄向尖晶石相转变压力为14 GPa, 与理论预测值(24 GPa)相比偏小.     

Electronic structures and hydrogen bond network of high-density and very high-density amorphous ices

Electronic structures of hexagonal ice (ice Ih), high-density amorphous ice (HDA), and very high-density amorphous ice (VHDA) are investigated using ab initio density functional theory (DFT) at 77 K under a pressure of 0.1 MPa, focusing on band structure, density of states (DOS), partial density of states (PDOS), and electron density. It is found that the integration intensity of the O-2p bonding band in HDA is 1.53 eV wider than that in the VHDA. Because more 2p electrons in HDA participate the 2p-1s hybridization of O-H. The classical molecular dynamics (MD) method has further been carried out to analyze the hydrogen bond network of HDA and VHDA with larger numbers of water molecules under the same temperature, pressure, and boundary conditions used as those during the DFT calculation. MD results show that there exists some water molecules with five hydrogen bonds in both HDA (4.1 +/- 0.1%) and VHDA (2.8 +/- 0.1%), as compared with the LDA, being consistent with the integration intensity results of PDOS. This result can be used to interpret the physical nature of the similar transition temperature of HDA and VHDA to LDA with different heating rates.     

Ultra-incompressible Ternary Diborides Re_（0.5）Ir_（0.5）B_2,Re_（0.5）Tc_（0.5）B_2,Os_（0.5）W_（0.5）B_2 and Os_（0.5）Ru_（0.5）B_2：Density Functional Calculation

In this paper, density functional computations have been applied to the structural, elastic and electronic properties of ternary transition metal diborides Re0.5Ir0.5B2, Re0.5Tc0.5B2, Os0.5W0.5B2 and Os0.5Ru0.5B2 in hexagonal (P63/mmc) and orthorhombic (Pmmn) structures with both local density approximation and generalized gradient approximation. LDA gives smaller lattice parameters and larger elastic moduli than GGA. Both results show that the hexagonal ones are more stable than orthorhombic ones except Os0.5Ru0.5B2. Moreover, the hexagonal structure has superior elastic property than orthorhombic one. Generally speaking, the calculated elastic moduli of Re0.5Ir0.5B2 and Os0.5Ru0.5B2 are smaller than those values of Re0.5Tc0.5B2 and Os0.5W0.5B2 within the same structure because of the filling of antibonding states. The relativistic effects result in weaker bonds of Tc-B (Ru-B) than those of Re-B (Os-B). All the diborides are ultra-incompressible. Re0.5Tc0.5B2 has the largest shear modulus and it is a promising superhard diboride like Os0.5W0.5B2. The elastic properties are in high correlation with the bond strength. The shear moduli are more sensitive than the bulk moduli to the bond strength.     

Simulation study of angle-resolved photoemission spectra and intramolecular energy-band dispersion of a poly(tetrafluoroethylene) oligomer film

Theoretical simulations of the angle-resolved ultraviolet photoemission spectra (ARUPS) for the oligomer of poly(tetrafluoroethylene) [(CF(2))(n); PTFE] were performed using the independent-atomic-center approximation combined with ab initio molecular orbital calculations. Previously observed normal-emission spectra for the end-on oriented sample (with long-chain axis perpendicular to the surface) showed the incident photon-energy (hnu) dependence due to the intramolecular energy-band dispersion along the one-dimensional chain, and the present simulations successfully reproduced this hnu dependence of the observed spectra. We employed the experimentally observed helical structure for PTFE oligomers for the simulations. We also calculated the density of states (DOS) for the planar zigzag structure, and examined the changes in the electronic structure due to the difference in the molecular structure by comparing the DOS for the helical and planar zigzag structures. Only a small change in the DOS was found between these structures, showing little change of the electronic structure between these conformations. We also evaluated the inner potential V(0), which is the parameter defining the energy origin of the free-electron-like final state, and checked the validity of the value of -10 eV estimated in our previous study using the experimentally observed hnu dependence of the peak intensity. The estimation of V(0) was performed by pursuing the best agreement between the energy-band dispersion [E=E(k)] relation along the chain direction obtained from the simulated spectra and the experimentally deduced one. An excellent agreement in the topmost band was achieved when the assumed inner potential V(0) was set at about zero. This value of V(0) is much different from the value of V(0)=-10 eV in the previous study, suggesting the invalidity of the previous assumption at the estimation of V(0) from the peak intensity variation with hnu. Using the presently obtained V(0), we could derive more reliable E=E(k) dispersion relation from the observed ARUPS spectra. The comparison of this newly derived relation gave good agreement with theoretically calculated E=E(k) relations, in contrast to the poor agreement for the previous results with V(0)=-10 eV.     

Structural and optical properties of passivated silicon nanoclusters with different shapes: a theoretical investigation

Optimized geometries and electronic structures of hydrogenated silicon nanoclusters, which include the Td and Ih symmetries, have been generated by using the semiempirical AM1 and PM3 methods, the density functional theory (DFT) B3LYP method with the 6-31G(d) and LANL2DZ basis sets from the Gaussian 03 package, and the local density functional approximation (LDA), which is implemented in the SIESTA package. The calculated diameters for these Td symmetric hydrogenated silicon nanoclusters are in the range from 6.61 A (Si5H12) to 23.24 A (Si281H172). For the Ih symmetry, we calculated Si20H20 and Si100H60 nanoclusters only. Theoretically, the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is size dependent. The calculated energy gap decreases (Si5H12: 7.65 eV to Si281H172: 3.06 eV) while the diameter of silicon nanocluster increases. By comparing different calculated results, we concluded that the calculated energy gap by B3LYP/6-31G(d)//LDA/SIESTA is close to that from experiment and that the LDA/SIESTA result underestimates the experimental value. On the contrary, the AM1 and PM3 results overestimate the experimental results. For investigation of the optical properties of Si nanoclusters as a function of surface passivation, we carried out a B3LYP/6-31G(d)//LDA/SIESTA calculation of the Si35 and Si47 core clusters with full alkyl-, OH-, NH2-, CH2NH2-, OCH3-, SH-, C3H6SH-, and CN- passivations. The calculated optical properties of alkyl passivated Si35 nanoclusters (Si35(CH3)36, Si35(C2H5)36, and Si35(C3H7)36) are close to one another and are higher than those of oxide, nitride, and sulfide passivated Si 35 clusters. In conclusion, the alkyl passivant affects weakly the calculated optical gaps, and the electron-withdrawing passivants generate a red-shift in the energy gap of silicon nanoclusters. A size-dependent effect is also observed for these passivated Si nanoclusters.     

Transition-metal 13-atom clusters assessed with solid and surface-biased functionals

First-principles density-functional theory studies have reported open structures based on the formation of double simple-cubic (DSC) arrangements for Ru(13), Rh(13), Os(13), and Ir(13), which can be considered an unexpected result as those elements crystallize in compact bulk structures such as the face-centered cubic and hexagonal close-packed lattices. In this work, we investigated with the projected augmented wave method the dependence of the lowest-energy structure on the local and semilocal exchange-correlation (xc) energy functionals employed in density-functional theory. We found that the local-density approximation (LDA) and generalized-gradient formulations with different treatment of the electronic inhomogeneities (PBE, PBEsol, and AM05) confirm the DSC configuration as the lowest-energy structure for the studied TM(13) clusters. A good agreement in the relative total energies are obtained even for structures with small energy differences, e.g., 0.10 eV. The employed xc functionals yield the same total magnetic moment for a given structure, i.e., the differences in the bond lengths do not affect the moments, which can be attributed to the atomic character of those clusters. Thus, at least for those systems, the differences among the LDA, PBE, PBEsol, and AM05 functionals are not large enough to yield qualitatively different results.