Terms Of Atomic Orbitals Research Articles

Two-dimensional GeSe monolayer doped with single main-group element atom for hazardous gas detection: A first-principles study

Currently, the identification of sensing materials that can selectively and rapidly respond to hazardous gases is urgent for safeguarding the environment and human life. The adsorption of hazardous gas molecules (NO, NO2, SO2, CO, and H2S) on the GeSe monolayer doped with single main-group element atom is investigated and compared based on first-principles density functional theory (DFT). NO2 has the highest adsorption energy and charge transfer adsorbed on both pristine GeSe and doped GeSe monolayer. At ambient temperature, for any doped GeSe monolayers adsorbing gas molecules (including H2O, O2, and N2), the occupation function value of NO2 is the highest. The analysis of density of states (DOS) and band structure indicates that NO2 interacts with different modified GeSe monolayers in terms of atomic orbitals. Moreover, Al-GeSe has the largest change of work function before and after the adsorption of NO2, meaning that Al-GeSe has the greatest response to NO2. current-voltage (I-V) characteristics demonstrate that Al-GeSe exhibits a remarkable current change after the adsorption of NO2. These results illustrate that Al-GeSe has a strong physical and chemical interaction with NO2, yielding a high response and selectivity, which is anticipated to be the optimal gas sensing material for NO2.

GronOR: Massively parallel and GPU-accelerated non-orthogonal configuration interaction for large molecular systems.

GronOR is a program package for non-orthogonal configuration interaction calculations for an electronic wave function built in terms of anti-symmetrized products of multi-configuration molecular fragment wave functions. The two-electron integrals that have to be processed may be expressed in terms of atomic orbitals or in terms of an orbital basis determined from the molecular orbitals of the fragments. The code has been specifically designed for execution on distributed memory massively parallel and Graphics Processing Unit (GPU)-accelerated computer architectures, using an MPI+OpenACC/OpenMP programming approach. The task-based execution model used in the implementation allows for linear scaling with the number of nodes on the largest pre-exascale architectures available, provides hardware fault resiliency, and enables effective execution on systems with distinct central processing unit-only and GPU-accelerated partitions. The code interfaces with existing multi-configuration electronic structure codes that provide optimized molecular fragment orbitals, configuration interaction coefficients, and the required integrals. Algorithm and implementation details, parallel and accelerated performance benchmarks, and an analysis of the sensitivity of the accuracy of results and computational performance to thresholds used in the calculations are presented.

Strain-induced effects on the electronic properties of 2D materials

Thanks to the ultrahigh flexibility of 2D materials and to their extreme sensitivity to applied strain, there is currently a strong interest in studying and understanding how their electronic properties can be modulated by applying a uniform or nonuniform strain. In this work, using density functional theory (DFT) calculations, we discuss how uniform biaxial strain affects the electronic properties, such as ionization potential, electron affinity, electronic gap, and work function, of different classes of 2D materials from X-enes to nitrides and transition metal dichalcogenides. The analysis of the states in terms of atomic orbitals allows to explain the observed trends and to highlight similarities and differences among the various materials. Moreover, the role of many-body effects on the predicted electronic properties is discussed in one of the studied systems. We show that the trends with strain, calculated at the GW level of approximation, are qualitatively similar to the DFT ones solely when there is no change in the character of the valence and conduction states near the gap.

ANALYSIS OF HISTORICAL APPROACHES TO THE PERIODIC TABLE IN GENERAL CHEMISTRY COLLEGE TEXTBOOKS USED IN BRAZIL IN THE TWENTIETH CENTURY

The aim of this paper was to analyze the historical approaches to the periodic table in general chemistry textbooks used in Brazilian universities in the twentieth century. Textbooks were qualitatively analyzed according to the following criteria: position of the discussion about the periodic table in the book; presence or absence of a specific chapter on the subject; presentation of attempts to classify chemical elements before Mendeleev; presentation of the construction process of the periodic table by Mendeleev; identification of problems in the original table; discussion of Mendeleev's predictions about unknown elements; organization of the periodic table; periodic properties presented and discussed; and the enunciation of the periodic law. The analysis revealed different approaches at specific periods of the twentieth century, from more descriptive approaches to emphasis on attempts to explain the periodic table in terms of atomic orbitals. These changes point to different ways of understanding chemistry during the period studied.

Ab initio electron energy‐loss spectra and depolarization effects: Application to carbon nanotubes

AbstractWe perform ab initio calculations of the optical absorption and electron energy‐loss spectra of (m, 0) single‐walled carbon nanotubes (with m ≠ 3n for m = 7‐25) in the framework of a “sum over states” (SOS) treatment of the Kohn‐Sham (KS) single‐particle orbitals and energies (CRYSTAL program). This approach tested on hexagonal boron nitride enables to fully assign the interband transitions in the imaginary part of the dielectric constant, in terms of atomic orbitals. As these calculations could not take into account the local field effects (depolarization effects), which take place for perpendicular polarizations in 2D and 1D periodic systems, we apply a simple method based on the Clausius‐Mossotti formula, relating the SOS and coupled‐perturbed KS polarizability values. This approach reproduces the main features of the spectra of boron nitride (001) surface and carbon nanotubes. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Relativistic effects on the nuclear magnetic shieldings of rare-gas atoms and halogen in hydrogen halides within relativistic polarization propagator theory

In this work an analysis of the electronic origin of relativistic effects on the isotropic dia- and paramagnetic contributions to the nuclear magnetic shielding sigma(X) for noble gases and heavy atoms of hydrogen halides is presented. All results were obtained within the 4-component polarization propagator formalism at different level of approach [random-phase approximation (RPA) and pure zeroth-order approximation (PZOA)], by using a local version of the DIRAC code. From the fact that calculations of diamagnetic contributions to sigma within RPA and PZOA approaches for HX(X=Br,I,At) and rare-gas atoms are quite close each to other and the finding that the diamagnetic part of the principal propagator at the PZOA level can be developed as a series [S(Delta)], it was found that there is a branch of negative-energy "virtual" excitations that contribute with more than 98% of the total diamagnetic value even for the heavier elements, namely, Xe, Rn, I, and At. It contains virtual negative-energy molecular-orbital states with energies between -2 mc2 and -4 mc2. This fact can explain the excellent performance of the linear response elimination of small component (LR-ESC) scheme for elements up to the fifth row in the Periodic Table. An analysis of the convergency of S(Delta) and its physical implications is given. It is also shown that the total contribution to relativistic effects of the innermost orbital (1s1/2) is by far the largest. For the paramagnetic contributions results at the RPA and PZOA approximations are similar only for rare-gas atoms. On the other hand, if the mass-correction contributions to sigma(p) are expressed in terms of atomic orbitals, a different pattern is found for 1s1/2 orbital contributions compared with all other s-type orbitals when the whole set of rare-gas atoms is considered.

Analytical and numerical development for the two-centre overlap-like quantum similarity integrals over Slater-type functions

In molecular quantum similarity measure between two molecules, using the molecular electron density, the major task involves the accurate numerical evaluation of overlap between the two electron densities in the integral measure. By expanding the electron densities in terms of atomic orbitals, using the linear combination of atomic orbitals approach, a large number of overlap-like quantum similarity integrals over the basis functions in the two molecules will be required accurately for the calculation of a meaningful quantum similarity measure. Improvement of the computational methods of these integrals would be indispensable to a further development in computational studies of large molecular systems. Analytic expressions were obtained for these overlap-like quantum similarity integrals over Slater-type functions, in terms of the usual two-centre overlap integrals over B functions. Different approaches were developed for the numerical evaluation of these two-centre overlap integrals over B functions, which can be used for the numerical evaluation of the two-centre overlap-like quantum similarity integrals over Slater-type functions. In this work we present our approach which is based on the use of nonlinear transformations for improving convergence of highly oscillatory integrals. In the case of more complicated multicentre integrals, this approach is shown to be able to produce remarkably good results. We also present different representations, which were obtained by Steinborn group, and which can be used for a fast and accurate computation of the two-centre overlap-like quantum similarity integrals over Slater-type functions.

Electronic structure of SnO2 (110) surface

The electronic structure of the stoichiometric and reduced SnO2 (110) surfaces is studied with first-principles calculations. Calculations are carried out with two complementary self-consistent ab initio–DFT–GGA methods. Surface relaxation is considered, where the most prominent feature turns out to be the surface layer in-plane oxygen displacement of the reduced surface outwards, about 0.4 Å with respect to the surface layer tin atoms. The electronic structure of the relaxed surfaces is considered in terms of atomic orbitals and rehybridization, and the surface band structure. The bands are flat at the stoichiometric surface, but strong dispersion occurs at the reduced surface. The dispersion results in electronic levels into the band gap, which have also been experimentally observed.

Coupled-channel calculations of the electronic energy loss

A review is given on the use of the coupled-channel method to calculate the electronic energy loss of ions penetrating matter. This first principle calculation, based on an expansion of the time-dependent electronic wave function in terms of atomic orbitals, has been applied to evaluate the impact parameter and angular dependence of the electronic energy loss and the stopping cross-section of ions (antiprotons, H and He) colliding with H and He atoms at energies of 1–500 keV/amu. The results are compared to experimental data as well as to others current energy-loss models.

Close-coupling calculations of transfer in collisions

We present calculations of the cross sections for muon transfer in collisions within the framework of the quantal close-coupling approximation. The total internal wavefunction of the system is expanded in terms of atomic orbitals on each atomic centre. The appropriate Jacobi coordinates are used in the direct and rearrangement channels. A pseudostate representation of the continuum is also employed. The t-matrix elements are obtained from the solution of coupled Lippmann - Schwinger equations using the momentum space formulation. For the isotopic exchange calculations the basis consists of a large number of s and p type atomic states symmetrically distributed between direct and rearrangement channels. The present results are in agreement with existing calculations in the case of elastic scattering for collision energies above about 0.001 eV. In the case of the rearrangement process the agreement is satisfactory for collision energies greater than 10 eV.

Close coupling calculation of alignment parameters in p + Mg(2p) collisions

A close coupling calculation based on expanding the wave functions in terms of atomic orbitals on the two collision centers is used to study the ionization and electron capture processes for p + Mg(2p) collisions. The alignment parameter of the resulting 2p hole is also calculated for the electron capture and the ionization processes. The results are compared with the experimental measurement and with the perturbation calculations.

Impact-parameter dependence of electronic energy loss and straggling of incident bare ions on H and He atoms by using the coupled-channel method.

A first-principles calculation based on an expansion of the time-dependent electronic wave function in terms of atomic orbitals (coupled-channel method) has been applied to evaluate the impact-parameter dependence of the electronic energy loss and the fluctuation in energy loss of swift ions colliding on H and He atoms at energies of 10 to 500 keV/amu. The results have been compared with experimental data as well as with other existing models, e.g., the local-density approximation in an electron-gas target, the harmonic-oscillator target treatment, and the first-order plane-wave-Born approximation. Our results show a nearly exponential shape of the mean electronic energy loss for small impact parameters, in contrast to the Gaussian shapes obtained by Mikkelsen and Sigmund [Nucl. Instrum. Methods B 27, 266 (1987)].

Redox properties of antitumour anthracyclines as predicted from ab initio calculations and electrochemical experiments

All-electron molecular orbital calculations were carried out on the aglycone moieties of the five antitumour anthracyclines daunomycin, 4-demethoxydaunomycin, 4-demethoxy-6-deoxydaunomycin, 4-demethoxy-11-deoxydaunomycin and 5-iminodaunomycin. Energy and composition (in terms of atomic orbitals) of the frontier molecular orbitals were compared with the redox properties resulting from electrochemical experiments. Four main conclusions were drawn: (a) the redox potential values of the chromophores can be anticipated, relative to a parent compound, from the HOMO—LUMO gap; (b) the energy of the HOMO is a good indicator of the propensity of anthracyclines to undergo oxidation; (c) the deglycosidation of anthracyclines actually depends on the strength of the bond being broken; (d) the ease of reductive activation of the substituted daunomycins seems to depend on the redox potential value, and therefore a molecule which is reduced more easily should interfere more efficiently in electron transfer processes and lead to a less pronounced production of activated oxygen species likely responsible for dangerous side effects like cardiotoxicity.

Angular and energy distributions of δ-rays ejected from low-z molecular targets by incident protons and α particles

The double differential cross-section mixed treatment (DDCS-MT), which has been developed in order to calculate the relatively energetic δ-rays measured along 4 to 12 MeV α particle tracks recorded in ionographic emulsions of various compositions, reproduces quite rigorously experimental DDCS for medium energy protons and α particles crossing not only helium and argon, but also molecular gases like water vapour, hydrocarbons, etc. In the latter case, a special procedure is required to describe the molecular target in terms of atomic quantities, notably by taking into account the binding energies of the molecular orbitals, as well as the breaking up of the molecular orbitals in terms of atomic orbitals.

Bonding in cubic and rhombohedral GeTe

The authors have investigated the bonding in cubic and rhombohedral GeTe by studying the charge density of each individual valence band, using the Baldereschi method. The calculated charge densities can be interpreted in terms of atomic orbitals. The orbitals that participate in the bonding are s Ge, p Te and p Ge. In cubic GeTe there are p orbitals along the three orthogonal axes forming polar sigma -bonds between the Ge and Te atoms. There is a pronounced admixture of s Ge states with p states in the second and the last valence band. This admixture is responsible for the rise of the last valence band, which makes the GeTe a narrow-gap semiconductor. For rhombohedral GeTe the bonding can be described in terms of one p orbital along the (111) direction and the three p orbitals orthogonal to the (111) direction. These orbitals interact and form an admixture of polar sigma - and pi -bonds. The bonding charge is greater for rhombohedral GeTe, resulting in an increase of the bonding and a lowering of the electronic energy of rhombohedral GeTe with respect to that of cubic GeTe. By calculation they find the dipole moment of the rhombohedral phase to be 1.4 D along the (111) direction.

CNDO/2 Calculations of α‐Quartz Electronic Properties of the Perfect Cluster and EPR Parameters of the O−; Vacancy

AbstractThe CNDO/2 method is applied is applied to a cluster cut out from a perfect α‐quartz crystal. The result is compared with that of an extended Hückel theory (EHT) calculation of the same cluster. The dominant character of valence band states in terms of atomic orbitals and the width of the energy gap are discussed. The formal charges of central cluster atoms are given. Furthermore EPR parameters of a relaxed O−; vacancy in α‐quartz are calculated on the basis of CNDO‐RHF and ‐ UHF approximations. The results are compared with those of an EHT calculation of the same model and with the experimental result of the E′1 centre in SiO2.

Effective convergence to complete orbital bases and to the atomic Hartree–Fock limit through systematic sequences of Gaussian primitives

Optimal starting points for expanding molecular orbitals in terms of atomic orbitals are the self-consistent-field orbitals of the free atoms and accurate information about the latter is essential for the construction of effective AO bases for molecular calculations. For expansions of atomic SCF orbitals in terms of Gaussian primitives, which are of particular interest for applications in polyatomic quantum chemistry, previous information has been limited in accuracy. In the present investigation a simple procedure is given for finding expansions of atomic self-consistent-field orbitals in terms of Gaussian primitives to arbitrarily high accuracy. The method furthermore opens the first avenue so far for approaching complete basis sets through systematic sequences of atomic orbitals. It is shown that, for expansions of atomic SCF orbitals in terms of even-tempered Gaussian primitives, the energy- optimized exponents are simple analytic functions of the number of primitives used in the expansions.With the help of these formulas, accurate atomic SCF calculations in terms of very large sets of primitives can be carried out without exponent optimization and the energies from such calculations converge to the Hartree–Fock limit on smooth curves. It proves possible to extrapolate to the limit and to give estimates for the error bounds of the extrapolation. Quantitative data are given for all atoms in the first three periods of the periodic system. The Hartree–Fock limits obtained in this manner are equal or superior to previous calculations based on numerical methods or on expansions in terms of exponential type primitives. From the given atomic SCF orbitals, extended basis sets for molecular calculations are easily obtained by Raffenetti’s method.

Energy partitioning theory for open shell systems at an INDO level

The theory of energy partitioning for open shell systems is formulated for the special case of 3 × 3 variational configuration interaction; the individual energy quantities are developed in terms of atomic orbitals. An energy analysis of the rotational barrier for the trimethylene biradical is presented.

On the mathematical formulation and interpretation of LACO MO theory

The mathematical basis of LCAO MO theory is studied, both within the Hartree-Fock approximation and in more exact formulations. The basic LCAO expansion for molecular orbitals ¦φ> in terms of atomic orbitals ¦x> is conveniently written ¦φ> = ¦x> S −1 B where S is the overlap matrix for atomic orbitals and B is the matrix of atomic orbital-molecular orbital overlaps. It is suggested that matrices P and Q, defined by P=B B† and Q=BnB† where n is the matrix of molecular orbital occupation numbers, are appropriate to the interpretation of molecular calculations in terms of atomic orbital components, electronic populations and the degree of bonding. Implications for Hartree-Fock calculations are investigated.

An application of the Pariser-Parr approximation to the nitrogen molecule

The semi-empirical method due to Pariser and Parr [1] is extended to make it applicable to an axially symmetrical molecule. Wave functions are expressed in terms of atomic orbitals before deriving expressions for the matrix elements of the effective Hamiltonian, instead of the more usual method of first expressing the matrix elements in terms of integrals involving molecular orbitals; this is found to facilitate the treatment, in that it enables us to make more direct use of atomic valence-state energies. An empirical formula is derived for a one-centre integral which does not occur in the more usual applications of the Pariser-Parr method. The method is used to calculate, for fixed nuclei, the energies, relative to the 1σg + ground term, of those six spectroscopic terms which can be obtained from the ground term by exciting one of the π u2p electrons to a π g2p orbital. Agreement with experiment is satisfactory, especially for those excited states which are not strongly affected by a neglected electronic configuration. Formulae are given for certain valence-state energies, additional to those earlier published.