First principles investigation of the electronic structure of the iron carbide cation, FeC+

We have studied 40 states of the diatomic iron carbide cation FeC(+) by multireference methods coupled with relatively large basis sets. For most of the states, we have constructed complete potential energy curves, reporting dissociation energies, usual spectroscopic parameters, and bonding mechanisms for the lowest of the studied states. The ground state is of (2)Delta symmetry, with the first excited state (a(4)Sigma(-)) lying 18 kcal/mol higher. The X(2)Delta state displays a triple-bond character, with an estimated D(0) value of 104 kcal/mol with respect to the adiabatic products or 87 kcal/mol with respect to the ground-state fragments.     

Oxidation of chromium carbide

The oxidation of chromium carbide has been studied gravimetrically. Products of reaction have been examined by gas sorption analysis and X-ray diffraction. Changes in phase composition, crystallinity and crystallite size are correlated with the reaction conditions.Chromium carbide, Cr₃C₂, differs from most of the transitional metal carbides in that it forms stable films of metal oxide (Cr₂O₃) around the remaining carbide particles, inhibiting further oxidation. Thus chromium carbide additive inhibits oxidation of interstitial zirconium carbide, ZrC, by forming some chromic oxide which stabilises the zirconia (ZrO₂) layer around the remaining carbide crystallites.     

First principles investigation on the ultra‐incompressible and hard TaN

The structural, electronic, and mechanical properties of TaN were investigated by use of the density functional theory (DFT). Eight structures were considered, i.e., hexagonal WC, TaN, NiAs, wurtzite, and CoSn structures, cubic NaCl, zinc‐blende and CsCl structures. The results indicate that TaN in TaN‐type structure is the most stable at ambient conditions among the considered structures. Above 5 GPa, TaN in WC‐type structure becomes energetically the most stable phase. They are also stable both thermodynamically and mechanically. TaN in WC‐type has the largest shear modulus 243 GPa and large bulk modulus 337 GPa among the considered structures. The volume compressibility is slightly larger than diamond, but smaller than c‐BN at pressures from 0 to 100 GPa. The compressibility along the c axis is smaller than the linear compressibility of both diamond and c‐BN. The estimated hardness is 34 GPa. Thus, TaN in WC‐type structure is a potential candidate to be ultra‐incompressible and hard. The unique mechanical properties of TaN in WC‐type structure would make it suitable for applications under extreme conditions. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

First principles investigation of vancomycin and teicoplanin binding to bacterial cell wall termini

The recent rise of vancomycin-resistant enterococci (VRE) has given new impetus to the study of the binding between glycopeptide antibiotics and bacterial cell wall termini. Here, we report on an extensive first principles investigation of the binding of vancomycin and teicoplanin with d-Ala-d-Lac (characteristic of VREs) and d-Ala-d-Ala (characteristic of non-VREs). Binding of both antibiotics to d-Ala-d-Ala was found to be stronger by about 3-5 kcal/mol and due primarily to the oxygen-oxygen lone-pair repulsion characteristic of the antibiotic/d-Ala-d-Lac complex. These results are in good agreement with recent experimental findings.     

First principles NMR calculations by fragmentation

The nuclear magnetic shielding tensor is a molecular property that can be computed from first principles. In this work we show that by utilizing the fragmentation approach, one is able to accurately compute this property for a large class of molecules. This is of great significance because the computational expense required in the evaluation of the shielding tensor for all nuclei in a large molecule is now subject to near linear scaling. On the basis of previous studies and this work, it is also very likely that all molecular properties that can be expressed as derivatives of the total energy of the system are also amenable to accurate evaluation via fragmentation. If only the chemical shifts for nuclei in a small part of a large molecule are of interest, then only those molecular fragments containing those nuclei need to have their shielding tensors evaluated. Further, the fragmentation approach allows one to construct a database of molecular fragments that could, in principle, be used in the NMR characterization of molecules and at the same time provide possible three-dimensional representations of these molecules.     

First principles molecular dynamics of molten NaCl

First principles Hellmann-Feynman molecular dynamics (HFMD) results for molten NaCl at a single state point are reported. The effect of induction forces on the structure and dynamics of the system is studied by comparison of the partial radial distribution functions and the velocity and force autocorrelation functions with those calculated from classical MD based on rigid-ion and shell-model potentials. The first principles results reproduce the main structural features of the molten salt observed experimentally, whereas they are incorrectly described by both rigid-ion and shell-model potentials. Moreover, HFMD Green-Kubo self-diffusion coefficients are in closer agreement with experimental data than those predicted by classical MD. A comprehensive discussion of MD results for molten NaCl based on different ab initio parametrized polarizable interionic potentials is also given.     

First principles study of magnetism in nanographenes

Magnetism in nanographenes [also known as polycyclic aromatic hydrocarbons (PAHs)] is studied with first principles density functional calculations. We find that an antiferromagnetic (AFM) phase appears as the PAH reaches a certain size. This AFM phase in PAHs has the same origin as the one in infinitely long zigzag-edged graphene nanoribbons, namely, from the localized electronic state at the zigzag edge. The smallest PAH still having an AFM ground state is identified. With increased length of the zigzag edge, PAHs approach an infinitely long ribbon in terms of (1) the energetic ordering and difference among the AFM, ferromagnetic, and nonmagnetic phases and (2) the average local magnetic moment at the zigzag edges. These PAHs serve as ideal targets for chemical synthesis of nanographenes that possess magnetic properties. Moreover, our calculations support the interpretation that experimentally observed magnetism in activated carbon fibers originates from the zigzag edges of the nanographenes.     

First principles semiclassical calculations of vibrational eigenfunctions

Vibrational eigenfunctions are calculated on-the-fly using semiclassical methods in conjunction with ab initio density functional theory classical trajectories. Various semiclassical approximations based on the time-dependent representation of the eigenfunctions are tested on an analytical potential describing the chemisorption of CO on Cu(100). Then, first principles semiclassical vibrational eigenfunctions are calculated for the CO(2) molecule and its accuracy evaluated. The multiple coherent states initial value representations semiclassical method recently developed by us has shown with only six ab initio trajectories to evaluate eigenvalues and eigenfunctions at the accuracy level of thousands trajectory semiclassical initial value representation simulations.     

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.     

First principles calculations on structure, bonding, and vibrational frequencies of SiP2

Pyrite type SiP(2) is reinvestigated by first principles calculations on various levels of functionals including local density approximation, generalized gradient approximation, Becke-Lee-Yang-Parr hybrid functional, and the Hartree-Fock method. SiP(2) is seen as a model compound with molecular [P-P] entities and [SiP(6)] octahedra. Structure and bonding are addressed by electronic structure calculations. Special attention is spent on P-P and Si-P bonds in terms of bond lengths and respective stretching modes from simulated Raman spectra. The electronic structure is analyzed in both direct and momentum space by the electron localization function and site projected density of states. The main goals of this work are to understand the nature of chemical bonding in SiP(2) and to compare and contrast the different methods of calculation.     

First principles molecular dynamics simulation of liquid rubidium

We present a first principles molecular dynamics simulation of liquid rubidium. The atomic forces are obtained from a quantum mechanical calculation of its electronic structure within the local density approximation of the density functional formalism and using the pseudopotential plane-wave method. We compare our results with the structure and dynamics predicted by classical molecular dynamics simulations.     

Chemical assembly of chromium oxide structures on the surface of disperse silicon carbide

The possibility was studied for synthesizing chromium oxide structures on the surface of K3–6 granular silicon carbide [GOST(State Standard 26327-84)] by its multiple successive treatment with chromium(VI) oxochloride, ethyl alcohol, and water vapors. The change in the chromium concentration in the modified samples was studied as a function of the number of successive treatment cycles. The samples were characterized by the X-ray phase analysis, X-ray photoelectron, IR, and electronic diffuse reflectance spectroscopies before and after the modification.     

Theoretical investigation of the ground and low-lying excited states of nickel carbide, NiC

The electronic structure and bonding of 19 states of the diatomic nickel carbide (NiC) has been studied by multireference methods. Potential energy curves have been constructed for all states, whereas for the three lowest states of symmetries X (1)Sigma(+), a (3)Pi, and A (1)Pi well separated from the rest of the states, special attention was paid through the use of very large basis sets and the calculation of core-valence correlation and scalar relativistic effects. The recommended binding energies for these states are 91, 67, and 54 kcal/mol with respect to the ground state atoms. Our results in general can be considered in fair agreement with the limited experimental findings.     

First principles study on the structural,electronic, and optical properties of Sc-doped AlN

Based on the density functional pseudo-potential method, the structural properties, the band structure, the density of states and the optical properties of the pure and Sc-doped AlN are calculated. The calculation results indicate that the defect of Sc(Al) exists steadily with a certain solubility in the doped system. Sc substitution of the Al site induces effective reduction of the band gap of AlN and the band gap being continuously reduced when increasing Sc concentrations. The existence of the strong hybridization between Sc 3d and N 2p indicates the transport of electrons from Sc atoms to N atoms. Besides, it is shown that the insertion of Sc atom leads to redshift of the optical absorption edge. The intensity of both the imaginary part of the dielectric function and the optical absorption of Al_{1 ? x} Sc _x N are found to decrease with increasing Sc concentrations in the low energy range.     

First principles determination of the bound levels of HeLi-

An analytical potential energy curve is developed from high quality ab initio calculations for the He+Li- interaction. The HeLi- electrostatic complex is found to have an Re of 18.5 bohrs and a De of 0.974 cm(-1). Numerical solution of the rovibrational Schr?dinger equation with this potential indicates two bound levels, (v,J)=(0,0) and (0,1), for all naturally occurring isotopologs (i.e., 4He7Li-, 4He6Li-, 3He7Li-, and 3He6Li-). For the common isotopolog, 4He7Li-, a D0 of 0.207 cm(-1) and an R0 of 26.5 bohrs is determined.     

First principles predictions of thermophysical properties of refrigerant mixtures

We present pair potentials for fluorinated methanes and their dimers with CO(2) based on ab initio potential energy surfaces. These potentials reproduce the experimental second virial coefficients of the pure fluorinated methanes and their mixtures with CO(2) without adjustment. Ab initio calculations on trimers are used to model the effects of nonadditive dispersion and induction. Simulations using these potentials reproduce the experimental phase-coexistence properties of CH(3)F within 10% over a wide range of temperatures. The phase coexistence curve of the mixture of CH(2)F(2) and CO(2) is reproduced with an error in the mole fractions of both phases of less than 0.1. The potentials described here are based entirely on ab initio calculations, with no empirical fits to improve the agreement with experiment.     

First principles studies on boron sites in zeolites

A systematic computational investigation on protonated and nonprotonated boron-containing zeolites (boralites), performed by using different periodic density functional theory approximations, is presented. Both minimum energy structures and finite temperature behavior of model boron sodalites were analyzed. All of the adopted computational schemes agree in predicting an acid site composed of a silanol Si-OH group loosely linked to a planar BO(3) structure in the protonated system and a BO(4) tetrahedral site in the sodium-containing zeolite. Calculated structural and vibrational properties are in line with experimental data. Comparisons of the protonated boralite site with Al and Ga zeolitic acid sites are discussed as well. Results indicate that this class of mild acid catalysts is characterized by significant framework flexibility and pronounced thermal effects due to the loosely bound acid site.     

First principles study of photostability within hydrogen-bonded amino acids

The photochemistry and photophysics of a two-glycine minimal model is studied at the CASPT2//CASSCF level of theory. Different photoinduced processes are discussed, on the basis of the calculated minimum energy paths and the characterization of the electronic state crossings. Two main processes could provide UV-photostability to the hydrogen-bonded peptide system: (i) forward-backward photoinduced electron/proton transfer involving the H in the hydrogen bond, (ii) singlet-singlet energy transfer between two amino acids, providing ultrafast population of the low-energy n,π* state.