Ab initio investigation of the CdTe (001) surface

Based on first-principles density functional pseudopotential calculations, the CdTe (001) surface has been studied. Atomic configurations of different reconstructions are obtained with good accuracy. This reveals the efficiency of the force calculation for the surface relaxations. The surface energies were calculated on relaxed surface slabs as a function of the chemical potentials. The main result is that the energy of the Te-terminated dimerized surface with a (2 × 1) reconstruction is larger than the Cd-terminated c(2×2) reconstructed surface. This is in agreement with what was suspected by the equilibrium model introduced by F. Tinjod et al., which explains the formation of the quantum dots in CdTe/ZnTe superstructures.     

Ab initio investigation of the Renner-Teller effect in tetra-atomic molecules

A method for theoretical ab initio treatment of the Renner-Teller effect in tetra-atomic molecules is described. It is based on the model developed in 1972 by Petelin and Kiselev, but instead perturbationally, as in the original work, the vibronic problem is solved by a variational approach. The reliability of the approximations on which the model is based is discussed in detail and checked by the explicit ab initio computations carried out at various levels of sophistication. The model is extended to take into account the interplay between the vibronic, spin-orbit and magnetic hyperfine couplings. The results of ab initio investigations of the structure of spectra involving the ground states, X²Π_u of C₂H⁺₂ and B₂H⁺₂ are reviewed.     

Ab initio predicted elastic and thermodynamic properties of Imm2-BN under high pressure

The structural parameters, elastic constants, thermodynamic properties of Imm2-BN under high pressure were calculated via the density functional theory in combination with quasi-harmonic Debye approach. The results showed that the pressure has the significant effect on the equilibrium lattice parameters, elastic and thermodynamic properties of Imm2-BN. The obtained ground state structural parameters are in good agreement with previous theoretical results. The elastic constants, elastic modulus, and elastic anisotropy were determined in the pressure range of 0–90?GPa. Furthermore, by analyzing the B/G ratio, the brittle/ductile behavior under high pressure is evaluated and the elastic anisotropy of the Imm2-BN up to 90?GPa is studied in detail. Moreover, the pressure and temperature dependence of thermal expansion coefficient, heat capacity, Debye temperature, and Grüneisen parameter are predicted in a wide pressure (0–90?GPa) and temperature (0–1600?K) ranges. The obtained results are expected to provide helpful guidance for the future synthesis and application of Imm2-BN.     

Ab initio study of the optical properties of crystalline phenanthrene,including the excitonic effects

Using the ab initio methods for solving the Bethe–Salpeter equation on the basis of the FPLAPW method, optical properties of crystalline phenanthrene were calculated, in a comparison to its isomer, anthracene. It was found that despite the similarity of the structural, electronic, and the overall optical properties in a 40 eV energy range, phenanthrene and anthracene show significant differences in their optical spectra in the energy range below band gaps. Phenanthrene has two spin singlet excitonic features whereas anthracene shows one. The singlet and the lowest triplet binding energies of phenanthrene were found to be larger than anthracene. In this study, in addition, a comparison has been made between the optical spectra in RPA and the existing experimental data.     

Ab initio calculations of structural,electronic, optical and thermodynamic properties of alkaline earth tellurides BaxSr1−XTe

Structural, electronic and thermodynamic properties of SrTe and BaTe compounds and their ternary mixed crystals Ba_xSr_1−xTe in the rock-salt structure have been studied with density functional theory (DFT), whereas the optical properties have been obtained by using empirical methods such as the modified Moss relation. The exchange-correlation potential was calculated using the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) and the local density approximation (LDA) of Teter–Pade (TP). In the present work, we used the virtual-crystal approximation (VCA) to study the effect of composition (x). The calculated lattice parameters at equilibrium volume and the bulk modulus for x=0 and x=1 are in good agreement with the literature data. Furthermore, the Ba_xSr_1−xTe alloys are found to be an indirect band gap semiconductor. In addition, we have also predicted the heat capacities (C_V), the entropy(S), the internal energy (U) and the Helmholtz free energy (F) of the parent compounds SrTe and BaTe.     

高压下Fe从bcc到hcp结构相变机理的第一性原理计算

采用基于密度泛函理论的第一性原理方法, 分别研究了压力作用下Fe从体心立方 (bcc,α 相) 结构到六角密排(hcp, ε相) 结构相变的两种不同的相变机理: 相变过程中出现亚稳定的面心立方(fcc) 结构(bcc-fcc-hcp) , 以及相变过程中没有出现亚稳定的fcc结构(bcc-hcp) . 计算结果表明: 静水压力条件下, 相变过程中并不会产生亚稳定的fcc结构, 这与最近的原位XRD实验测量结果相一致. 随着压力的增加, fcc-hcp的相变势垒逐渐增加, 压力趋向于阻止Fe从fcc结构到hcp结构的相变. 计算得到了相变过程中原子磁性和结构的详细信息, 分析表明相变过程中涉及复杂的磁性转变, 并且讨论了原子磁性对结构转变影响的物理机理. 此外, 对分子动力学模拟中产生亚稳定的fcc结构的原因也进行了讨论. 关键词： 相变机理 静水压力 第一性原理 铁     

Ab initio calculations of CO physisorption on ceria(1 1 1)

Applying the method of increments, we have performed MP2 and CCSD(T) calculations for the physisorption of CO on a cerium site on the ceria(1 1 1) surface. Our calculations predict an interaction energy of −0.28 eV. We have compared our calculations to previous CCSD(T) calculations for the physisorption of CO on a cerium site on the ceria(1 1 0) surface and found a difference in the interaction energy that is related to the different structure of the two surfaces. On the ceria(110) surface only 30% of the interaction energy originate from electron correlation effects, but on the ceria(111) surface almost the entire binding energy (80%) is due to electron correlation effects. Analyses of the interaction energy contributions show that most of the electron correlation part originates from the interaction of CO with the O ions in the topmost surface layer.     

Ab initio studies of hydrogen desorption from low index magnesium hydride surface

The low index Magnesium hydride surfaces, MgH₂(0 0 1) and MgH₂(1 1 0), have been studied by ab intio Density Functional Theory (DFT) calculations. It was found that the MgH₂(1 1 0) surface is more stable than MgH₂(0 0 1) surface, which is in good agreement with the experimental observation. The H₂ desorption barriers vary depending on the crystalline surfaces that are exposed and also the specific H atom sites involved - they are found to be generally high, due to the thermodynamic stability of the MgH₂ system, and are larger for the MgH₂(0 0 1) surface. The pathway for recombinative desorption of one in-plane and one bridging H atom from the MgH₂(1 1 0) surface was found to be the lowest energy barrier amongst those computed (172 KJ/mol) and is in good agreement with the experimental estimates.     

Ab initio insights on photophysics of 9-methylhypoxanthine

In this work, the low-lying electronic singlet states of 9-methylhypoxanthine (9MHPX) were explored by the complete active space self-consistent-field (CASSCF) and complete active space second-order perturbation theory (CASPT2) calculations, and the conical intersections between the optically bright excited S₁ state and ground S₀ state were optimised by the two-root state-averaged SA-2-CASSCF approach. These studies indicate that four slightly different kinds of S₁/S₀ conical intersections are identified computationally for 9MHPX, corresponding to four main internal conversion pathways, respectively, all of which are found to show the comparable timescales according to dynamics simulations. At the CASPT2 level, four bright ππ* transitions of 9MHPX are calculated to locate at 4.47, 5.35, 5.97 and 6.30 eV, respectively, responsible for the available experimental absorption peaks of 9MHPX in the vapour phase (4.41, 5.19, 6.05 and 6.42 eV). Though one relatively weak ππ* transition computed at 5.69 eV is not observed in the vapour phase, it is in accordance with the circular dichroism measurement of another hypoxanthine derivative deoxyinosine 5'-phosphate near 5.51 eV.     

Ab initio calculation of transport properties in 1,3-diphenylpropynylidene based molecular device

Using an ab initio method based on non-equilibrium Green’s functions (NEGF) combined with density functional theory (DFT), a calculation of the transport properties of a single molecular junction based on 1,3-diphenylpropynylidene (PhC₃Ph) ‘radical-π-radical’ is performed. The obvious negative differential resistance (NDR), spin current polarisation (SCP) and dual-spin current rectification (SCR) effects in this device are obtained. The total current for magnetic parallel configuration (PC) is larger at first and then less than that for magnetic antiparallel configuration (APC) as the bias increases, which suggests the abnormal magnetoresistance (MR) effect and can be used as a molecular switch with two working voltages. The evolution of the spin-polarised transmission spectrums and the frontier molecular orbitals (MOs) with applied bias is used to explain the above interesting results. Our calculations may be helpful for designing multifunctional molecular spintronics devices in the future.     

Fourier transform emission spectroscopy and ab initio calculations on NbCl

The emission spectrum of NbCl has been recorded in the 3000-20 000 cm⁻¹ region using a Fourier transform spectrometer. The bands were observed by microwave excitation of a mixture of NbCl₅ vapor and He. Two groups of bands observed in the 6500-7000 cm⁻¹ and 9800-11 000 cm⁻¹ regions have been assigned to two electronic transitions. Five bands observed in the 6500-7000 cm⁻¹ region consist of R, P, and Q branches with no combination defect or Λ-doubling. They have been assigned as five sub-bands of a ΔΛ=±1 transition with Λ>1. Nine bands observed in the 9800-11 000 cm⁻¹ regions consist of R and P branches, and they are also free from Λ-doubling. These bands have been classified into four sub-bands of a ΔΛ=0 transition (with Λ>1), which has tentatively been assigned as . The two transitions have no electronic states in common. Ab initio calculations have been performed on NbCl and the spectroscopic properties of the low-lying electronic states have been calculated. The ground state of NbCl has been predicted to be a state arising from the 3σ¹ 1δ² 2π¹ configuration, with a low-lying state at 1300 cm⁻¹ from the 3σ¹ 1δ¹ 2π² configuration. The results of our experimental and theoretical studies will be presented. This work represents the first experimental investigation of the spectra of NbCl and the first ab initio prediction of the spectroscopic properties of the low-lying electronic states.     

Ab initio calculations of the stability and lattice dynamics of the MgSiO3 post-perovskite

Using the first-principles computations we have calculated phonon-dispersion relations and the phonon density of states for perovskite and post-perovskite phases of MgSiO₃. From them using the quasiharmonic approximation we have estimated the free energies, bulk modulus and volume expansion coefficients of both structures at various pressures and temperatures. Our calculations indicate that the thermal expansivity of MgSiO₃ changes very little across the phase transition. We have determined the P-T coexisting line of perovskite and post-perovskite phases.     

Structural stabilities and electronic structures of Ti atomic chains

The present first principles density-functional calculations reveal that titanium can form one-dimensional chains in linear, dimer, zigzag and ladder structures. The most stable structure is a zigzag chain with a unit cell rather close to an equilateral triangular geometry with four nearest neighbors. Two intermediary chains between the linear and zigzag ones have the ladder and dimer structure, respectively. Titanium can also form a metastable zigzag structure with an obtuse bond angle. It is important and interesting to find that during the elongation of the zigzag chain, the bond angle will shift abruptly from a sharp angle to an obtuse one at a critical point, and the bonding character also transforms from mainly metallic to more covalent. This is the first time that such a structural transition is discussed in the atomic chain system. The electronic structures of these one-dimensional titanium chains are also discussed.     

第一性原理计算Fe从bcc到hcp结构的相变路径及其磁性边界

本文采用基于密度泛函理论的第一性原理方法,计算了压力作用下Fe从bcc到hcp结构相变的势能面、相变路径以及相变过程中的磁性相边界.结果表明:与Burgers路径不同,相变过程中bcc结构(110)bcc面的剪切和相对滑移相互耦合,并伴随有(110)bcc面间距的减小;这一相变机制可以解释Fe的静高压实验中在相变初期观察到的hcp结构异常.因此,并不需要像Wang和Ingalls提出的那样,在相变过程中引入一个亚稳定的fcc相来解释这些实验结果.对相变势能面的计算表明剪切对相变的发生有激活作用.此外,分析表明相变过程中涉及复杂的磁性转变,相变过渡态位置正好位于磁性相边界上,并对原子磁性对结构转变影响的物理机制进行了讨论.     

Ab initio calculations study of the electronic, optical and thermodynamic properties of NaMgH3, for hydrogen storage

The structural stability, electronic structure, optical and thermodynamic properties of NaMgH₃ have been investigated using the density functional theory. Good agreement is obtained for the bulk crystal structure using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy. It is found from the electronic density of states (DOS) that the valence band is dominated by the hydrogen atoms while the conduction band is dominated by Na and Mg empty states. Also, the DOS reveals that NaMgH₃ is a large gap insulator with direct band gap 3.4 eV. We have investigated the optical response of NaMgH₃ in partial band to band contributions and the theoretical optical spectrum is presented and discussed in this study. Optical response calculation suggests that the imaginary part of dielectric function spectra is assigned to be the interband transition. The formation energy for NaMgH₃ is investigated along different reaction pathways. We compare and discuss our result with the measured and calculated enthalpies of formation found in the literature.     

Ab initio chemisorption studies of H on Fe(110)

Ab initio configuration interaction calculations are performed to study the chemisorption of atomic H on a Fe(110) surface. The lattice is modeled as an embedded three-layer, 40-atom cluster with the Fe atoms fixed at the bulk position. Fe 3d orbitals are explicitly included on five Fe atoms on the surface. Hydrogen strongly binds to the Fe(110) surface at the long-bridge, short-bridge, and quasi three-fold sites. The calculated adsorption energies are 2.76, 2.73, and 2.71 eV, respectively. H-surface bonding at the on-top Fe site is more than 0.4 eV weaker. The calculated H-surface distances are 0.89, 1.03, and 0.87 Å for H at the long-bridge, short-bridge, and quasi three-fold sites, respectively, which agrees well with the LEED value of 0.9 ± 0.1 Å. The H-surface stretching vibrational frequencies are calculated to be 1070, 1066, and 1073 cm⁻¹, at the long-bridge, short-bridge, and quasi three-fold sites, respectively. The work function of Fe(110) decreases on H adsorption. The present calculations indicate that H diffusion into the bulk through the short-bridge site will have a much higher activation barrier than via the long-bridge and quasi three-fold sites.     

Ab initio potential energy surface for the nitrogen molecule pair and thermophysical properties of nitrogen gas

A four-dimensional potential energy hypersurface (PES) for the interaction of two rigid nitrogen molecules was determined from high-level quantum-chemical ab initio computations. A total of 408 points for 26 distinct angular configurations were calculated utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory and basis sets up to aug-cc-pV5Z supplemented with bond functions. The calculated interaction energies were extrapolated to the complete basis set limit and complemented by corrections for core–core and core–valence correlations, relativistic effects and higher coupled-cluster levels up to CCSDT(Q). An analytical site–site potential function with five sites per nitrogen molecule was fitted to the interaction energies. The PES was validated by computing second and third pressure virial coefficients as well as shear viscosity and thermal conductivity in the dilute-gas limit. An improved PES was obtained by scaling the CCSDT(Q) corrections for all 408 points by a constant factor, leading to quantitative agreement with the most accurate experimental values of the second virial coefficient over a wide temperature range. The comparison with the best experimental data for shear viscosity shows that the values computed with the improved PES are too low by about 0.3% between 300 and 700?K. For thermal conductivity large systematic deviations are found above 500?K between the calculated values and most of the experimental data.