Ab initio study on structural and electronic properties of BanOm clusters

Density-functional calculation within local density approximation, shows that the electronic property of a barium oxide cluster is strongly correlated with its equilibrium structure. The ground-state structures of BanOm (4 < or = n < or = 9,m < or = n) clusters can be classified into four categories: (a) compact, (b) dangling state, (c) F-center, and (d) stoichiometric. The compact cluster is metallic, almost no energy gap exists between the highest occupied and the lowest unoccupied molecular orbitals. The energy gap for the dangling state cluster is larger than that for the F-center cluster, while the stoichiometric cluster has the largest energy gap.     

Ab initio study of structural and electronic properties of zigzag graphene nanoribbons on hexagonal boron nitride

Results of the study of structural and electronic properties of the 8-ZGNR/h-BN(001) heterostructure by the pseudopotential method using plane waves within density functional theory are presented. Within one approximation the features of the spin state at the Fermi level are studied along with the role of the edge and substrate effects in the opening of the energy gap in the 8-ZGNR/h-BN(001) heterostructure in both ferromagnetic and antiferromagnetic orderings. The effect of a substrate made of hexagonal boron nitride was found for the first time. It consists in the opening of the energy gap in the π electron spectrum of the 8-ZGNR/h-BN(001) heterostructure for the ferromagnetic spin ordering. It is shown that the gap was 30 meV. Contributions of the edge effects of the graphene nanoribbon and the substrate to the energy gap formation are differentiated for the first time. It is found that in the 8-ZGNR/h-BN(001) heterostructure the dominant role in the opening of the energy gap at the Fermi level is played by the edge effects. However, when the nanoribbon width decreases, e.g., to six dimmers the substrate role in the gap opening increases and amounts to 45%. Local magnetic moments of carbon atoms are estimated. It is shown that small magnetic moments are induced on boron and nitrogen atoms at the interface.     

Ab initio study of structural and electronic properties of SrTiO3 (001) oxygen-vacancy surfaces

First-principles calculations are employed to study the surface relaxation and electronic structure of the fully relaxed SrTiO(3) (001) oxygen-vacancy surfaces with both Sr and Ti terminations. In contrast to the perfect surface, the larger surface rumples and smaller interlayer distances have been found. Some in-gap Ti 3d states at about -1.13 eV below the Fermi level were observed in the Ti-terminated surface caused by oxygen vacancies. For the Sr-terminated oxygen-vacancy surface, some in-gap Ti 3d states move into the bulk midgap region to become partially occupied. These theoretical results are in agreement with the experimental data.     

Ab initio study of the electronic structure of the crystalline high-mobility organic semiconductor 1,4-diiodobenzene

We use ab initio crystalline calculations to explore the role of iodine in determining the properties of 1,4-diiodobenzene. The results strongly suggest that the large halogen is an important or perhaps the dominant determinant of the ability of diiodobenzene to transport holes. We conjecture that the high mobility of diiodobenzene is due to a combination of electronic and phononic effects, both influenced by the presence of the iodine.     

Ab initio study on the electronic and mechanical properties of ReB and ReC

The structural, electronic, and mechanical properties of ReB and ReC have been studied by use of the density functional theory. For each compound, six structures are considered, i.e., hexagonal WC, NiAs, wurtzite, cubic NaCl, CsCl, and zinc-blende type structures. The results indicate that for ReB and ReC, WC type structure is energetically the most stable among the considered structures, followed by NiAs type structure. ReB-WC (i.e., ReB in WC type structure) and ReB-NiAs are both thermodynamically and mechanically stable. ReC-WC and ReC-NiAs are mechanically stable and becomes thermodynamically stable above 35 and 55 GPa, respectively. The estimated hardness from shear modulus is 34 GPa for ReB-WC, 28 GPa for ReB-NiAs, 35 GPa for ReC-WC and 37 GPa for ReC-NiAs, indicating that they are potential candidates to be ultra-incompressible and hard materials.     

Ab initio calculations of structural, elastic, and electronic properties of silver nitrides

The all-electron approach implemented in the CRYSTAL06 program is used along with a pseudopotential method in the pseudo-atomic orbital basis set to study the crystal structure, elastic constants and bulk moduli, the band structure and density of states for the family of silver nitrides. Calculations are performed within density functional theory with the use of local and gradient functionals to describe exchange and correlation. For the general type of the cubic lattice, all considered compounds can be put in the following order of their relative stability: AgN (rock salt structure), AgN₂ (fluorite structure), Ag₂N (cuprite structure), and Ag₃N (anti-ReO₂). It is shown that AgN, AgN₂, and Ag₂N are metals, whereas Ag₃N is a semiconductor with a band gap of 0.25 eV. Chemical bonding in these compounds has ionic and covalent components, apart from the metal one.     

Ab initio study on the structural and electronic properties of Li3GaP2 compared with Li3GaN2

Structural and electronic properties of Li₃GaP₂ and Li₃GaN₂ have been investigated by the first-principles calculations within the density functional theory. The calculated lattice parameters of the two compounds are in excellent agreement with the available experimental data. Both Li₃GaP₂ and Li₃GaN₂ are direct band gap semiconductors with the band gaps of 1.26 eV and 2.37 eV, respectively. The Ga–P (Ga–N) and Li–P bonds consist of a mixture of ionic character and covalent nature, while the Li–N bond exhibits almost ionic. The bonds in the Li₃GaP₂ are shown to have stronger covalency and weaker ionicity as compared to the corresponding ones in the Li₃GaN₂.     

Ab initio studies of structural and electronic properties of Li4Ti5O12 spinel

Structural and electronic properties of Li₄Ti₅O₁₂ spinel are studied from density functional theory based first principles calculations. Differences on these properties between delithiated state Li₄Ti₅O₁₂ and lithiated state Li₇Ti₅O₁₂ are compared. The optimized lattice constant of Li₄Ti₅O₁₂ is 8.619 Å, which is even a little larger (0.2%) than 8.604 Å of the lithiated state Li₇Ti₅O₁₂. The arrangement of the Li and Ti atoms at the 16d sites of the spinel structure is also investigated in a cubic unit cell. Large 1 × 1 × 3 supercell models are constructed and used to calculate the total energy and electronic structure. The average intercalation potential is also calculated, with metallic lithium as reference.     

Ab initio calculations of structural,electronic, and optical properties of Cu2HgSnSe4

The electronic structure of Cu₂HgSnSe₄ in stannite structure is studied by the first-principles calculations. This material is a direct band-gap compound. In addition, the dielectric function, absorption coefficient, reflectivity, and energy-loss function are studied using the density functional theory within the generalized gradient approximation. We discuss the optical transitions between the valence bands and the conduction bands in the spectra of the imaginary part of the dielectric function at length. High and wide optical absorption spectrum is obtained. There are several prominent peaks for Cu₂HgSnSe₄ in the reflectivity spectra. And a rapid decrease of reflectance corresponds to the prominent peak in the energy-loss spectrum.     

Ab initio study of the electronic structure of manganese carbide

We report electronic structure calculations on 13 states of the experimentally unknown manganese carbide (MnC) using standard multireference configuration interaction (MRCI) methods coupled with high quality basis sets. For all states considered we have constructed full potential energy curves and calculated zero point energies. The X state, correlating to ground state atoms, is of 4sigma- symmetry featuring three bonds, with a recommended dissociation energy of D0 = 70.0 kcal/mol and r(e) = 1.640 angstroms. The first and second excited states, which also correlate to ground state atoms, are of 6sigma- and 8sigma- symmetry, respectively, and lie 17.7 and 28.2 kcal/mol above the X state at the MRCI level of theory.     

Ab initio study of the electronic spectrum of peroxyacetyl nitrate

A theoretical study of the ground and excited states of peroxyacetyl nitrate (PAN), CH3C(O)OONO2, has been carried out using high level ab initio molecular orbital methods. The ground state geometry and vibrational frequencies are calculated using the coupled-cluster method. The vertical excitation energies for the lowest three excited states are calculated using the complete active space self-consistent field method along with the multireference internally contracted configuration interaction method. These results are compared with vertical excitation energies calculated with the coupled cluster equation of motion method. The calculation provides relevant insight into the origin of PAN absorption in the UV wavelength region from 200 to 300 nm. The nature of the electron transitions for these excited states is discussed.     

Ab initio study of sphere-like mesogen properties

Sphere-like compound C8H17Si(OPhC12H25)3N (1) forms mesophases. In order to investigate the relationship between molecular structure and liquid crystal properties, structural studies are carried out on the model molecules of compound 1 and its substituted derivatives using ah initio calculations. The results show that the cyano or chloro substituted tribenzosilatrane compounds R1Si(OPhR2 )3N (R1 R2 = CN or Cl) have much bigger dipole moments or anisotropy of polarizability and more like sphere than the corresponding alkyl substituted compounds. Cyano or chloro substituted tribenzosilatranes would be better candidates for sphere-like meso-gens.     

Ab initio SCF CL study of the electronic spectrum of nitroso-methane

Configuration interaction studies of ground, n_ → π^*, n₊ → π^*, and π → π^* electronically excited states are reported for nitroso-methane in its eclipsed equilibrium geometry. The first (n_ → π^*) and the second (n⁺ → π^*) singlet states are calculated at 2.17 and 7.14 eV. it is shown that a significant delocalization of the nonbonding orbitals on the nitrogen and oxygen is responsible for the large energy gap between these two states. The two lowest triplet states occur at 1.29 and 5.39 eV and are of n_ → π^* and π → π^* origin.     

Ab initio study of the electronic structure and bonding of aluminum nitride

For the diatomic aluminum nitride (AlN), we have constructed potential energy curves for 45 states employing multi-reference variational methods and quantitative basis sets. Thirty-six states are relatively strongly bound, five present local minima, and four are of repulsive nature. Almost all states are of intense multi-reference character rendering their calculation and interpretation quite problematic. Our tentative assignment of the ground state is 3Pi, while a 3Sigma- state is above by less than 1 kcal/mol. Our best estimate for the binding energy of the X3Pi state is D0 = 56.0 +/- 0.5 kcal/mol at re = 1.783 A, in good agreement with the experimental values of D = 66 +/- 9 kcal/mol and re = 1.7864 A. The binding energy of the A3Sigma- state is very similar to the X state because they both correlate to the ground-state atoms, but the bond distance of the former is 0.13 A longer. The first seven states can be tagged as follows: X3Pi, A3Sigma-, a1Sigma+, b1Pi, c1Delta, B3Sigma+, and d1Sigma+, a rather definitive order with the exception of X and A states.     

Ab initio analysis of the structural properties of alkyl-substituted polyhedral oligomeric silsesquioxanes

Ab initio quantum mechanical calculations have been performed to establish the potentials for alkyl-substituted polyhedral oligomeric silsesquioxane (POSS) monomers RxH8-x(SiO1.5)8. More specifically, we have examined the unsubstituted POSS (SiO1.5H)8 cage as well as linear and cyclic alkyl-substituted cages where one of the terminating hydrogen atoms is replaced by a hydrocarbon group, that is, R1H7(SiO1.5)8. The results for the minimum-energy configurations indicate that the presence of the linear hydrocarbon chains and cyclic intermediates have very little effect on the structure of the POSS cage. Although the POSS monomeric cage does influence the partial charges of the first few carbon atoms covalently bound to the POSS monomer, its effect on the structural properties of the alkyl chain is small. Differences arise, however, for cyclic alkyl substitutents bound to the POSS cage due to the repulsive interactions between the POSS cage and bulkier cyclic intermediates that result upon rotation of the Si-C-C-C dihedral angles. The interatomic potentials for these rotational, or torsional, terms need to be modified slightly in order to appropriately simulate sterically hindered substitutents on the cage. Our results suggest that combining an atomistic force field independently developed to describe silsesquioxanes with an independent atomistic model developed to describe hydrocarbon chains can be used in classical molecular simulation studies of most alkyl-silsesquioxanes. This avoids the need to develop specific force fields for each substituted POSS cage studied and opens up the possibility of using molecular simulation to probe the thermodynamic and structural properties of these unique nanoscale building blocks.     

Ab initio study of the low lying electronic states of ZnF and ZnF-

Highly correlated ab initio calculations have been performed for an accurate determination of the electronic structure and of the spectroscopy of the low lying electronic states of the ZnF system. Using effective core pseudopotentials and aug-cc-pVQZ basis sets for both atoms, the potential curves, the dipole moment functions, and the transition dipole moments between relevant electronic states have been calculated at the multireference-configuration-interaction level. The spectroscopic constants calculated for the X(2)Sigma(+) ground state are in good agreement with the most recent theoretical and experimental values. It is shown that, besides the X(2)Sigma(+) ground state, the B(2)Sigma(+), the C(2)Pi, and the D(2)Sigma(+) states are bound. The A(2)Pi state, which has been mentioned in previous works, is not bound but its potential presents a shoulder in the Franck-Condon region of the X(2)Sigma(+) ground state. All of the low lying quartet states are found to be repulsive. The absorption transitions from the v=0 level of the X(2)Sigma(+) ground state toward the three bound states have been evaluated and the spectra are presented. The potential energy of the ZnF(-) molecular anion has been determined in the vicinity of its equilibrium geometry and the electronic affinity of ZnF (EA=1.843 eV with the zero energy point correction) has been calculated in agreement with the photoelectron spectroscopy experiments.     

Ab initio SCF and CI study of the electronic spectrum of pyridine N-oxide

Configuration interaction (CI) studies of ground, n *, * * electronically excited states are reported for pyridine N-oxide. The transition energy to the lowest * excited ¹ B ₂ state is calculated at 4.35 eV, compared to the experimental spectrum range of 3.67–4.0 eV. This state lies below the lowest n * excited ¹ A ₂ state calculated at 4.81 eV above the ground state. The only experimentally reported triplet state at 2.92 eV above the ground state is predicted to be the ³ A ₁ (*) state. The calculated energy lies at 3.27 eV. Numerous other high-lying singlet states as well as the triplet states have also been calculated. The intramolecular charge transfer character of the ground and the excited states have been studied in terms of the calculated dipole moment and other physical properties.     

Ab initio study of low-lying electronic states of the FNO2 molecule

Ab initio electronic structure calculations are reported for low-lying electronic states, ¹A₁, ¹A₂, ³A₂, ¹B₁, ³B₁, ¹B₂, and ³B₂ of the FNO₂ molecule. Geometric parameters for the ground state ¹A₁ are predicted by MRSDCI calculations with a double-zeta plus polarization basis set. The vertical excitation energies for these electronic states are determined using MRSDCI/DZ+P calculations at the ground-state equilibrium conformation. The oscillator strengths and radiative lifetimes for some electronic states are calculated based on the MRSDCI wave functions. © 1993 John Wiley & Sons, Inc.     

Ab initio study of low-lying electronic states of the PF2 radical

The equilibrium geometries, excitation energies, force constants, and vibrational frequencies of the low-lying electronic states X²B₁, ²A₁, ²B₂, and ²A₂ of the PF₂ radical have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated geometry, force constants, and vibrational frequencies for the X²B₁ state are in good agreement with experimental data. The electronic transition moments, oscillator strengths for the ²A₁ → X²B₁ and ²A₂ → X²B₁ transitions, and radiative lifetimes for the ²A₁ and ²A₂ states are calculated based on the MRSDCI wave functions. © 1994 by John Wiley & Sons, Inc.     

Ab initio configuration-interaction study of the ground and low-lying electronic states of NiI

For the first time, we have studied the potential-energy curves, spectroscopic terms, vibrational levels, and the spectroscopic constants of the ground and low-lying excited states of NiI by employing the complete active space self-consistent-field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified six low-lying electronic states of NiI with doublet spin multiplicities, including three states of Delta symmetry and three states of Pi symmetry of the molecule within 15 000 cm(-1). The lowest (2)Delta state is identified as the ground state of NiI, and the lowest (2)Pi state is found at 2174.56 cm(-1) above it. These results fully support the previous conclusion of the observed spectra although our computational energy separation of the two states is obviously larger than that of the experimental values. The present calculations show that the low-lying excited states [13.9] (2)Pi and [14.6] (2)Delta are 3 (2)Pi and 3 (2)Delta electronic states of NiI, respectively. Our computed spectroscopic terms, vibrational levels, and spectroscopic constants for them are in good agreement with the experimental data available at present. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally.