bluuebunny/crossref_metadata_2025_split huggingface dataset:
Acetaldehyde photochemistry: Testing the additivity assumption and polarized basis set effects
Authors: Yadav J. S. , Goddard J. D. 
Published: December 2004

AbstractSelected portions of the S0 and T1 potential energy surfaces of acetaldehyde surveyed in our earlier studies have been reexamined. The assumption of the additivity of basis‐set polarization and of electron correlation effects used extensively in our earlier work on acetaldehyde has been tested through explicit polarized basis‐set electron‐correlation calculations. The “additivity assumption” introduces average absolute errors in energy differences of only 1.9 (MP3) to 3.4 (MP2) kcal mol−1 in seven comparisons. The effects of using 6‒31G** SCF optimized geometries as opposed to single‐point calculations on 3‒21G SCF structures (6–31G**//3–21G) as in our previous papers were examined. In six comparisons, the average absolute error in relative SCF energies introduced by the use of the 3–21G geometries rather than the fully consistent 6–31G– ones was only 0.3 kcal mole−1. After a uniform scaling procedure, comparisons of the 6–31G** and 3–21G calculated vibrational frequencies with experiment for CH3CHO (S0), CH4, and CO (20 comparisons) yielded absolute differences of 41 cm−1 (6–31G**) and 57 cm−1 (3–21G). All these more elaborate calculations support for the specific case of acetaldehyde and various minima and transition states of relevance to its photochemistry, the commonly used and practically important approximations (e.g., additivity) made in our earlier studies.
Ab initio calculations of phenylene ring motions in polyphenylene oxide
Authors: Laskowski Bernard C. , Jaffe Richard L. , Komornicki Andrew 
Published: December 2004

AbstractAn important class of polymers is comprised, in part, of 1,4‐disubstituted phenylene groups. It is widely believed that large amplitude phenylene torsional motions play a critical role in determining such physical properties as the toughness and degree of crystallinity of these polymers. We have studied what is perhaps the simplest polymer in this class, polyphenylene oxide (PPO), using ab initio quantum chemistry methods to determine the conformational properties, torsional potential energy functions, and vibrational frequencies. From our calculations on dimer (diphenyl ether) and trimer (para‐diphenoxy benzene) fragments emerges a qualitative description of the mechanism of phenylene rotation in the polymeric material.
Analysis of nonadditivity effects and estimation of many‐body effects in linear water chains
Authors: Fülscher M. P. , Mehler E. L. 
Published: December 2004

AbstractCalculations have been carried out on small linear water chains in order to investigate nonadditivity and cooperativity effects and to study the rate of convergence of the many‐body interaction potential. Ab initio wave functions were computed for the pentamer and all related subgroups by using the self‐consistent, nonorthogonal group function approximation. The interaction energy and its electrostatic, exchange, polarization, and charge‐transfer components were decomposed into a sum of n‐body terms. Simple formulas were developed for estimating the interaction energy of higher‐order complexes, which also allow an extrapolation to be made to infinite chain length. Comparison of the extrapolated interaction energies, with and without including the nonadditivity effects, showed that the latter increases the H‐bond's stability by more than 25%. The results further suggest that the interaction potential can be truncated at either the three‐ or four‐body level, depending on the accuracy required, but that, at best, only rough qualitative results can be expected when pair additivity is assumed. Finally, the dipole moment appears to be sufficiently converged with the inclusion of trimeric terms.
Ab initio calculations on the quenching of excited lithium atom by molecular hydrogen. I
Authors: Matsumoto Shiro , Mizutani Kimiko , Sekiguchi Atsushi , Yano Takakazu , Toyama Masayuki 
Published: December 2004

AbstractMCSCF calculations were performed on the two lowest states of the LiH2 system, using a basis set of double‐zeta‐plus‐polarization quality. The characters of the wave functions in the region of the conical intersection of the potential energy surfaces were investigated in some detail. The depth of the minimum on the Ã2B2 surface was estimated to be 0.46 eV. The frequency of the a1 vibration at the center of the conical intersection was 3.76 X 1013 sec−1. Under the symmetry of the nuclear geometry lower than C2v in the immediate neighborhood of the center of the intersection, the upper state was characterized by a 3a′ natural orbital by which the more distant H is bonded to Li, while in the lower state the nearer H is bonded to Li. Thus, the process of the nonadiabatic transition caused by a nuclear motion of b2 symmetry is visualized as a smooth bond‐preserving process, and is expected to occur with high probability. For the correct description of the states in this region, MCSCF functions comprising limited CSF's were inadequate, and an additional process of nonorthogonal CI was required.
A theoretical examination of substituent effects on the detoxification reaction between glutathione and halogenated methanes
Authors: Luke Brian T. , Loew Gilda H. , McLean A. D. 
Published: December 2004

AbstractAn important metabolic pathway for halogenated methanes is the detoxification reaction with glutathione in the cell cytosol fraction. Experimental studies have shown that the rate of this SN2 displacement reaction is directly related to the leaving group ability of the dissociating halide; the rate increases in order of the ions F−, Cl−, Br−, and I− leaving. In this study, we examine the role of the other halomethane substituents on the rate of this reaction for compounds with a common leaving anion. To this end, reaction Cl− + CY3Cl → ClCY3 + Cl− (Y = H,F,Cl) is examined using ab initio methods. The barrier for this exchange process may arise from requiring an inversion of the CY3 group in the transition structure, the energy required to homolytically cleave the CCl bond, or from unfavorable steric and electronic interactions in the five‐coordinated transition structure. Of these three factors, only the third explains the calculated ordering of barrier heights. This suggests that successive halogenation of methane not only increases its activity as a substrate for anaerobic reduction, as was shown earlier, but also decreases its ability to take part in the detoxification reaction with glutathione.
Hydrogen chemisorption on, and diffusion through, palladium clusters
Authors: Baykara N. A. , Andzelm J. , Salahub D. R. , Baykara S. Z. 
Published: December 2004

AbstractLCGTO‐VWN‐MP calculations have been performed for PdH, Pd3H, and for a series of Pd10H clusters representing hydrogen chemisorbed on a (111) surface and at various sites along a diffusion path through the cluster. The calculated chemisorption geometry (over a threefold site) has a PdH distance of 1.72 Å, slightly longer than that proposed by Eberhardt et al. (1.69 Å) on the basis of comparison of photoemission data with slab calculations. Agreement with known energy differences is very good: We find the chemisorption site 0.25 eV (exp. 0.26) more stable than the most stable (octahedral) bulk site. The top of the diffusion barrier, a triangular site, is calculated 0.34 eV (exp. 0.23) above the octahedral site. All the sites have a split‐off state of Pd d, H s character below the d bands, indicating that the ‘invisible’ state of H/Pd(111) recently (not) observed in UPS cannot straightforwardly be attributed to hydrogen incorporation at sites such as these.
On the role of symmetry in the ab initio hartree‐fock linear‐combination‐of‐atomic‐orbitals treatment of periodic systems
Authors: Dovesi Roberto 
Published: December 2004

AbstractThe symmetry properties of the mono‐ and bielectronic terms contributing to the Fock matrix in the ab initio Hartree–Fock treatment of periodic systems are discussed. A computational scheme which takes full advantage of the point symmetry is presented; in this respect, it represents a generalization of the scheme proposed in Int. J. Quantum Chem. 17, 501 (1980). Computational details and numerical examples are reported; it is shown that with respect to two of the bottlenecks of this kind of calculation, namely, computer time and backing storage required for the bielectronic integrals, it is possible to obtain saving factors as large as h and h2, respectively, where h is the order of the point group. Preliminary tests are reported which indicate that the study of relatively complicated systems, like quartz or corundum (9 and 10 atoms in the unit cell, respectively) at an ab initio Hartree–Fock level is now within reach.
On weighted löwdin orthogonalization
Authors: Bhatia Rajendra , Mukherjea Kalyan K. 
Published: December 2004

AbstractLet f1, ⃛, fn be a basis of unit vectors and g1, ⃛, gn an orthonormal basis in a Hilbert space. We consider and solve the problem of finding an orthonormal basis e1, ⃛, en such that a weighted average of the distance of ej from fj and gj is minimized in the sense of least squares.
Structures and 1H‐chemical shifts of conjugation deficient hydrocarbons
Authors: Vogler H. 
Published: December 2004

AbstractThe geometrical and electronic structures and the 1H‐chemical shifts of some conjugation deficient hydrocarbons and their benzenoid isomers 1–3 which consist of two concentric perimeters have been studied by means of semiempirical procedures. Conclusions on the importance of macrocyclic annulenoid conjugation are drawn from the bond lengths pattern and the proton shifts in these compounds.
Electron correlation forms in the LiH molecule—fermi correlation
Authors: Feng Xuejun , Dahl Jens Peder 
Published: December 2004

AbstractCorrelation forms for electrons of the same spin small molecules are analyzed by taking the LiH molecule as an example. The analyses are carried out on the basis of the reduced density matrix theory and for both the ground state and the first excited state. It turns out that although there is a certain similarity, the correlated motions of parallel‐spin electrons in this four‐electron molecule are more complicated than those found for the two‐electron molecule. The correlation forms are not always constant. They vary essentially from one state to another, but are insensitive to fairly large changes of the internuclear distance for a given state.
Near‐Hartree–Fock wave functions for solids: The case of crystalline silicon
Authors: Pisani C. , Dovesi R. , Orlando R. 
Published: December 2004

AbstractSilicon is taken as a test system for assessing present‐day feasibility of calculations for crystalline solids of near‐Hartree–Fock quality. The calculations have been performed using CRYSTAL, an ab initio Hartree–Fock crystalline‐orbital LCAO program for periodic systems. The influence of the computational parameters that control the truncation of infinite sums on the results has been investigated; it is shown that a reasonable accuracy (numerical errors on total energy per atom below 10−3 a.u.) can be obtained while keeping the computational burden within manageable limits. The effect on the results of basis‐set size and quality is discussed. A number of basis sets have been tested, from minimal to relatively extended sets (28 atomic orbitals per atom). The quality of the wave function has been checked using variational criteria and also through a comparison with experimental data, such as equilibrium geometry, bulk modulus, electron charge density, and electron momentum distribution. For the latter quantities, which are a measure of the accuracy of the one‐electron density matrix, the best basis sets provide agreement with experiment that is almost within the experimental error. The correlation energy has been estimated using nonlocal density functionals, based on the one‐electron density matrix: After this correction, the atomization energy agrees with experiment to within 2%. The generalization of the above analysis to other crystals is briefly discussed.
Structural and electronic properties of infinite cis and trans polyenes: Perturbation theory of electron correlation effects
Authors: Suhai S. 
Published: December 2004

AbstractThe geometrical structure of five infinite polyene models (equidistant and alternating trans, equidistant cis, cis‐transoid, and trans‐cisoid) have been optimized using three different atomic basis sets at the Hartree–Fock level and by including electron correlation effects within the second order of Møller–Plesset perturbation theory. The single‐particle energy bands have also been corrected for correlation effects applying the electron polaron method. Bond alternation has always reduced the energy (both for trans and cis) polyenes but this stabilization energy has decreased with an increasing basis set and has saturated about 3 mH for trans and trans‐cisoid and about 7.5 mH for the cis‐transoid model. On the absolute energy scale, however, the alternating trans structure has always turned out to be more stable than either the cis‐transoid or the trans‐cisoid one. The energetic order is in all cases trans < cis‐transoid < trans‐cisoid. Using the largest basis set with correlation, the corresponding energy differences are ΔE(trans → cis‐transoid) = 1.69 mH and ΔE(trans → trans‐cisoid) = 6.12 mH, respectively. The HF values of the valence‐band widths vary in the region of 4–6 eV for the cis models and about 7 eV for the trans one. Correlation reduced them by about 20%, and the centers of the bands were shifted upward by about 2 eV due to self‐energy renormalization. The best values for the electron polaron valence bandwidths are 6.4, 4.2, 3.3, and 5.2 eV for the alternating trans, trans‐cisoid, cis‐transoid, and equidistant cis models, respectively. The vertical ionization potentials of the above four structures are 4.80, 5.47, 5.71, and 4.18 eV, respectively. The conduction bands have a width of 6–8 eV at the HF level (except equidistant cis with 4.6 eV for the larger basis sets) that will be reduced by about 15–20% due to polaron formation. The bands are shifted in this case uniformly downward by about 2–3 eV due to correlation. The best values obtained for the conduction bandwidths are 4.7, 6.4, 5.6, and 3.7 eV, respectively. These band shifts reduce the HF value of the fundamental gap by several eV's for all models. For the largest basis set, the ΔEgap values change from 6.6, 6.9, 7.7, and 5.3 eV (HF results for the alternating trans, trans‐cisoid, cis‐transoid, and equidistant cis models, respectively) to 2.7, 3.2, 4.5, and 2 eV, respectively, at the correlated level.
Molecular orbital cluster model study of bonding and vibrations of CO adsorbed on MgO surface
Authors: Pacchioni Gianfranco , Cogliandro Giuseppe , Bagus Paul S. 
Published: December 2004

AbstractThe interaction of CO with the MgO(100) surface has been investigated by means of all electron cluster model calculations. The CO molecule is bound on the Mg2+ site of MgO with a chemisorption energy of about 0.2 eV. The binding mechanism is electrostatic in nature and arises almost entirely from the interaction of the weak electric field generated by the ionic surface and the CO charge distribution, with negligible contributions from chemical effects as the CO σ donation. When CO is bound through carbon, its vibrational frequency increases with respect to the gas‐phase value. This shift, Δ, has been analyzed and decomposed into the sum of different contributions. It is found that the positive Δω does not arise entirely from the field–dipole interaction but is due, in part, to the increase in Pauli repulsion occurring when the CO molecule vibrates in the presence of the surface “wall.” A stronger electrostatic interaction, bringing the CO adsorbate closer to the surface, increases this wall effect and results in a more pronounced positive ω shift. It is also found that the two CO orientations exhibit opposite shifts in ωe, thus, the two orientations can be distinguished, in principle, by IR spectroscopy. The analysis of our ab initio cluster wave functions gives a very different picture than the standard view of the metal–CO bond as arising from σ donation and π back donation.
On the interaction of ions with a platinum metal surface
Authors: Seitz‐Beywl J. , Poxleitner M. , Probst M. M. , Heinzinger K. 
Published: December 2004

AbstractAb initio calculations have been performed to obtain analytical potential functions for describing the interactions between a platinum surface and both lithium and iodide ions. The accuracy of the results is tested by model calculations with large basis sets and the inclusion of correlation energy. The potentials obtained are to be used in computer simulations of electrochemical phenomena near solid–liquid interfaces.
Ab initio force field for simulations of proteins and nucleic acids
Authors: Aida Misako , Corongiu Giorgina , Clementi Enrico 
Published: December 2004

AbstractWe present a new self‐consistent set of ab initio analytical pair potential to predict specific nonbonded interactions of protein with nucleic acid, of protein with protein, and of nucleic acid with nucleic acid. The purpose of this study is to represent the interaction between biological molecules with an accuracy equivalent to the ab initio molecular orbital calculations, which are used as reference data to obtain the pair potentials. Atoms in nucleic acids and proteins are classified according to their chemical environments. An “effective charge,” a modification of a charge obtained from the Mulliken population analysis, is introduced and used to represent the electrostatic energy. More than 30,000 SCF interaction energies have been calculated to provide the reference data for the fitting procedure that we have adopted in the parameterization of the potentials. The standard deviation is 1.61 kcal/mol for interaction energies spanning the range from about −220 kcal/mol to +20 kcal/mol. Molecular dynamics simulations, using the above new set of force field, have been performed successfully for the systems where adequate treatments of specific interactions are required: The stability of α‐helix of C‐peptide and the interaction of spermine with oligonucleotide are examined as preliminary examples.
Calculation of 13C‐NMR chemical shift using the intermediate neglect of differential overlap model
Authors: Baker John David , Zerner Michael C. 
Published: December 2004

AbstractThe local origin/local orbital (LORG) method of Hansen and Bouman has been implemented with the intermediate neglect of differential overlap Hamiltonian for spectroscopy (INDO/S). The method is shown capable of demonstrating the inductive effects associated with electron‐withdrawing substituents through the diamagnetic shielding term. In addition, the method is capable of differentiating chemical shift in differing bond environments. The calculated paramagnetic contribution, however, is deficient for substituents that saturate the minimal basis such as oxygen and fluorine, which severely limits the general utility of the procedure. Through the utilization of reduced linear equations for the paramagnetic term, the method is amenable to any molecule for which a self‐consistent field can be performed and therefore can potentially be used to study very large systems. At present, however, the LORG method when used with the rapid INDO/S model Hamiltonian does not reliably reproduce the paramagnetic contribution to the shielding.
Molecular aspects of superconductivity: The role of the one‐particle term at the gap formation
Authors: Svrček Michal , Baňacký Pavol , Zajac Anton 
Published: December 2004

AbstractAn exact solution for the electron‐vibrational problem of the nonadiabatic molecular system has been obtained. By the quasi‐particle transformation technique, the fermionic Hamiltonian has been derived and solved at the ab initio level. Results clearly and unambiguously show that the gap formation due to nonadiabatic electron–phonon coupling is mediated by the one‐particle electron–phonon interaction term, whereas the two‐particle one represents just a correction to the correlation energy. The temperature dependence of the gap and electronic specific heat connected with the electron–phonon coupling have also been derived.
Novel pentacoordinated bridged silicon anions
Authors: Davis Larry P. , Burggraf Larry W. , Gordon Mark S. 
Published: December 2004

AbstractBoth ab initio and semiempirical electronic structure calculations are used to investigate the molecular and electronic structures and eneregetic stabilities of an unusual bridged compound with the general formula [YSiH3XSiH3Y]−, with Y = H or F and X = H, CH3, NH2, OH, F, or Cl. Most of these bridged anions are quite stable relative to YSiH3 + XSiH3Y−, and the stability is predicted to increase considerably when Y = H is replaced with Y = F.
To(Γ) phonons in GaAs under uniaxial strain
Authors: Lino A. T. , Meskini N. , Alves H. W. L. , Kunc K. 
Published: December 2004

AbstractUsing the density‐functional theory in the local‐density approximation, norm‐conserving pseudopotentials, and plane‐wave expansions, we have developed a simple way to calculate ab initio the uniaxial strain dependence of the TO(Γ) phonon mode in GaAs, including the envolved relaxations effects. The relaxation response of the system was evaluated from the use of the elasticity theory. Our results confirm the experimental evidence and show a reasonable agreement with the observed data for the frequency splitting of this phonon mode. © 1993 John Wiley & Sons, Inc.
Variational principle for transition matrix
Authors: Titov A. V. 
Published: December 2004

AbstractA variational principle for the transition matrix is considered. Conception of the MC SCF (in particular, CASSCF) approach for the transition matrix is proposed as a solution of the variational problem on the optimal evolution of a packet of two stationary states with some additional conditions. An ordinary MC SCF method for a single state is a special case of the proposed approach. Some aspects of the solving of the equations for the optimal transition matrix are treated. The method can be used in atomic and molecular calculations of transition energies, oscillator strengths, and other properties. © 1993 John Wiley & Sons, Inc.
Application of the GCA to a series of curve crossing topologies
Authors: Broeckhove J. , Keutgens W. , Lathouwers L. , Van Leuven P. 
Published: December 2004

AbstractThe generator coordinate approximation (GCA) for the calculation of spectra of diatomics is briefly reviewed. Results of this method applied to a series of curve crossings are presented. In all cases, the generator coordinate method performs significantly better than does the adiabatic approximation. When the energy gap becomes smaller, the GCA results improve. In the crossing limit, the GCA becomes exact. © 1994 John Wiley & Sons, Inc.
Theoretical study of the conjugation length effect on the electronic and second‐order nonlinear optical properties of amino–nitro diphenylacetylenes
Authors: Dehu C. , Brédas J. L. 
Published: December 2004

AbstractWe presnet a theoretical study of the effect of the conjugation length on the electronic properties and second‐order molecular polarizabilities β in p‐amino‐p′‐nitrodiphenylacetylene molecules where the number of triple bonds in the conjugated segment varies from 1 to 4. The β values are calculated via an intermediate neglect of differential overlap/single configuration interaction (INDO/SCI) sum‐over‐states (SOS) approach. We test the convergence of the SOS method and the validity of the two‐state model to describe the β response. The results indicate that increasing the conjugation length results in a decrease of the charge transfer within the molecule. The two‐state model is shown to break down as the conjugated segment extends to four triple bonds; this is due to the appearance of several low‐lying nearly isoenergetic excited states that significantly contribute to the β response. The theoretical results are in excellent agreement with recent experimental data. © 1994 John Wiley & Sons, Inc.