Ab Initio Configuration Interaction Calculations Research Articles

Electron spin resonance investigation of Sc+2 in neon matrices and assignment of its ground electronic state as X 4Σ−. Comparison with theoretical calculations

The discandium radical cation, Sc+2, has been isolated in neon matrices at 4 K and studied by electron spin resonance (ESR) spectroscopy and theoretical methods. It was produced by the x-irradiation of neon matrix samples containing neutral Sc2 which was formed by trapping the products generated from the pulsed laser vaporization of scandium metal. The experimental and theoretical findings indicate that Sc+2 has a 4Σ−g electronic ground state compared to an X 5Σ−u state for the neutral discandium radical. The large decrease in the 45Sc hyperfine interaction (A tensor) going from Sc2 to Sc+2 provides direct experimental information concerning the types of valence molecular orbitals that are involved in these diatomic radicals. The neon matrix magnetic parameters for 45Sc+2 are g∥≊2.00, g⊥=1.960(1), ‖A∥‖=28(6), and ‖A⊥‖=26.1(3) MHz; the D value (zero field splitting) was 15 381(3) MHz. Ab initio configuration interaction (CI) calculations of the nuclear hyperfine interactions yielded results in reasonable agreement with the experimental observations.

Use of relativistic core potentials to compute potential curves and lifetimes of low-lying states of arsenic fluoride

The effects of spin—orbit coupling and other relativistic interactions on the electronic structure of the arsenic fluoride molecule are analyzed with the aid of ab initio configuration interaction calculations based on effective core potentials. The computed results for potential curves. vibrational wavefunctions and radiative transition probabilities between various low-lying states of AsF are compared with analogous data for the fluorides of the heavier Group V A atoms, antimony and bismuth. Measured spin— orbit splittings for the X 3Σ − and A 3Π λ-s states are accurately reproduced in the relativistic Cl and computed trends in the parallel and perpendicular b0 +-X transition probabilities for this group of fluorides are found to agree with the observations of Colin, Herman and Prevot. Inclusion of a semidiffuse s orbital on the fluorine atom is important in describing the σ* MO occupied in the A 3Π and c 1Π λ-s states. Computed electric dipole moments indicate that the latter states are more strongly polarized (negative F) than their lower-energy counterparts with π* 2 electronic configurations.

Ab initio calculations of the core ionization spectra of unsaturated hydrocarbons

Ab initio configuration interaction (CI) calculations have been performed relative to the core ionization spectra of ethylene, propene, trans-butadiene and trans-hexatriene. The capability of the various CI schemes employed has been pointed out, in particular the 2h-1p CI, which is the simplest one and is attractive for its wide range of application, is able to give a qualitative reproduction of the spectra. For the description of the strong correlation effects present in these molecules more extended schemes are needed and semiquantitative results are obtained through the definition of a valence space with a projection scheme in which double excitations are generated. In this way the shape of the spectra is nicely reproduced, with a decisive improvement over previous calculations and a more detailed analysis of the effects which come into play is obtained. Furthermore some general trends of the spectra of more extended systems can be anticipated on the basis of the present results.

Assignment of the vacuum-ultraviolet spectrum of ethylene sulphide

Based on ab initio configuration interaction calculations and the previously measured vacuum-ultraviolet spectrum of ethylene sulphide, we have been able to assign essentially all absorption peaks, including the s, p and d Rydberg series up to n =9.

Excited electronic states of protonated acetylene. II. Anisotropy of the potential energy surface

Extensive ab initio configuration interaction calculations have been carried out for different directions of approach of a proton to a rigid acetylene molecule. The calculated potential energy curves of several singlet states are presented. The anisotropy we found allows us to explain features of the charge transfer mechanism during proton–acetylene collisions as observed in recent molecular beam experiments. An important theoretical conclusion is the preference of the linear approach of protons in collisions with acetylene for the formation of acetylene ions.

Valence—Rydberg mixing in the excited doublet states of HCO: potential surfaces for HCO separation

Valence—Rydberg mixing of the doublet electronic states of HCO along the HCO dissociation path is investigated by means of ab initio configuration interaction calculations. The character of the species in question and the form of the corresponding potential surfaces are discussed in terms of composition and energy changes of the molecular orbitals, generated in SCF computations, with geometry variation.

Theoretical investigations on thallium halides: Relativistic and electron correlation effects in T1X and T1X3 compounds (XF, C1, Br, and I)

AbstractRelativistic and electron correlation effects in thallium halides TlX and TlX3 (XF, Cl, Br, and I) are investigated by extensive ab initio configuration interaction calculations. Spin–orbit coupling is included at the Hartree–Fock level for the diatomic TlBr and TlI. At the best level of treatment of electron correlation (quadratic configuration interaction), the calculated molecular properties are in good agreement with experimental results, i.e., for the diatomic thallium halides deviations from experimental values are <0.06 Å for bond distances, <0.14 mdyn/Å for force constants, <35 kJ/mol for dissociation energies, and <0.3 D for dipole moments. The convergence of the Møller–Plesset series up to the fourth order is discussed. Two alternative structures of TlI3 are compared. At the Møller–Plesset level of theory, the trigonal planar structure with thallium in the oxidation state + 3 is the preferred gas phase arrangement compared with the bent arrangement containing a linear I unit and thallium in the oxidation state + 1, the difference being ca. 95 kJ/mol. Vibrational frequencies are predicted for all trigonal planar thallium(III) halides. © 1993 John Wiley & Sons, Inc.

Multireference configuration interaction calculation of the potential energy curves for OH bond breaking in the ground and lowest excited states of the water monomer and dimer

Abstract Ab initio multireference configuration interaction (MRCI) calculations of OH bond breaking in the water monomer and dimer are reported in this paper. Several basis sets and MRCI methodologies are tested in order to assess the quality of the expected results. Calculation of the hydrogen transfer curves in the dimer leads to the proposition that the coupling of excited states of A ″ symmetry — correlating to the state C 1 B 1 in the equilibrium geometry of the monomer—causes decomposition of the dimer into HO √ and H 3 O √ when it is attempted to obtain its resonance enhanced multiphoton ionization spectrum under similar conditions to those used for the water monomer.

Reaction of CH 4 with substitutional Fe/Ni(111)

The reaction of CH 4 with a substitutional Fe/Ni(111) surface is treated using a many-electron embedding theory, modelling the lattice as a 41-atom, three layer cluster. Ab initio valence orbital configuration interaction (multiple parent) calculations carried out on a local surface region permit an accurate description of bonding at the surface. The 3d orbitals are explicitly used for the Fe atom and six nearest neighbor Ni atoms in the local surface region. The calculated activation energy for CH 4 dissociation at an atop Fe site to produce CH 3 and H coadsorbed on the surface is 5.7 kcal/mol, compared with 16.7 kcal/mol on Ni(111). The reaction of CH 4 → CH 3(ads) + H(ads) is predicted to be 4 kcal/mol exothermic on the Fe/Ni(111) surface and the immediate dissociation products are CH 3 and H coadsorbed at across atom three-fold sites. Across bond dissociation is energetically very unfavorable. The bonding properties of coadsorbed CH 3 and H on the Fe/Ni(111) surface are similar to those on the Ni(111) surface. Compared to the clean Ni(111), the substitutional Fe/Ni(111) surface is very effective in activating CH 4.

A quantum mechanical study of the active site of aspartic proteinases.

We have performed ab initio self-consistent field (SCF) and configuration interaction (CI) calculations on the active site of the aspartic proteinases pepsin and endothiapepsin. The active site, which carries a formal negative charge to effect hydrolysis, was modeled as a formic acid/formate anion moiety and a water molecule, and the nearest hydrogen bonding residues (Gly34, Ser35, Gly217, and Thr218, with respect to the residue numbering in endothiapepsin) were modeled as formamide and methanol molecules. Four possible binding modes for the active-site water molecule were considered. In contrast to previous theoretical studies, we predict that the most stable form has the water molecule forming a bifurcated hydrogen bond to the inner oxygens of Asp32 and -215, with Asp32 being ionized. The calculations suggest that the water molecule prefers to bind across the shortest OD32 ... OD215 diagonal of the active-site carboxyl groups and therefore the binding mode of the water molecule for all the native aspartic proteinases can be readily predicted by measuring these distances.

Dissociative adsorption of H2 on Ni(111)

Ab initio configuration interaction calculations are performed to study the dissociative adsorption of H2 on a Ni(111) surface. The lattice is modeled as an embedded three-layer 41-atom cluster. Ni 3d orbitals are explicitly included on seven Ni atoms on the surface. H is preferentially chemisorbed at a threefold site on Ni(111) and the calculated binding energy of 62 kcal/mol, H–Ni distance of 1.86 Å, and H vibrational frequency of 1176 cm−1 are in excellent agreement with experimental data. H adsorbed at bridge and on-top Ni sites is 2.5 and 8.1 kcal/mol less stable, respectively. The heat of reaction H2 (gas)→2 H (ads) is calculated to be 22.0 kcal/mol exothermic. When two H atoms are adsorbed as nearest neighbors to the same Ni atom, threefold sites are preferred with H atoms adsorbed at fcc–fcc, hcp–hcp, or across atom fcc–hcp sites. These structures are consistent with the observed (2×2)−2H low energy electron diffraction pattern. The average adsorption energy per H is calculated to be 62 kcal/mol for the across atom case. Adsorption of H at a threefold site on Ni(111) has a tendency to block adsorption at adjacent threefold sites; coadsorbed H atoms at adjacent sites are unbound by 13.5 kcal/mol relative to H2 at infinite separation. For three H atoms chemisorbed at separated threefold sites sharing a Ni atom with the equal H–H lateral distances of 2.48 Å, the calculated average adsorption energy is 60 kcal/mol per H. Dissociation of H2 at an on-top Ni site has the lowest activation barrier of 1.6 kcal/mol. For the transition state, the H–H bond length, H-surface height, and H–H vibrational frequency are 1.22 Å, 1.38 Å, and 2506 cm−1, respectively. Molecular H2 adsorbed over an on-top site is bound by 3.4 kcal/mol with a slightly stretched H–H bond length of 0.79 Å, H-surface height of 1.85 Å, and H–H stretching frequency of 3396 cm−1.

Surface chemical reactions studied via ab initio-derived molecular dynamics simulations: Fluorine etching of Si(100)

Previous isothermal dynamics simulations of the interaction of F with Si(100) failed to predict any reaction beyond saturation of the surface dangling bonds. We show that this lack of reactivity was due to the overly repulsive nature of the empirical potential employed. We used the method of simulated annealing to fit a new analytic interaction potential to data from ab initio configuration interaction calculations. This potential was then utilized in isothermal molecular dynamics simulations to explore the mechanism by which fluorine begins to etch silicon. Calculated adsorption and reaction probabilities, as a function of both fluorine coverage and structure, reveal that the buildup of the fluorosilyl layer occurs via several competing reactions and that it does not follow a well defined reaction sequence. This competition creates disorder in the adsorbed fluorosilyl layer, which is shown to be an important precursor to continued reaction. Idealized ordered surface structures are shown to be unstable relative to highly disordered structures for coverages of more than 1.25 ML of fluorine.

Dynamical couplings of the predissociative n = 3 triplet gerade complex of H 2

We have performed ab initio configuration interaction calculations for the states involved in the n = 3 triplet gerade complex of H 2. The corresponding radial and rotational couplings needed in collisional studies have been evaluated analytically. Their physical origin, dynamical implications and comparison with previous data are discussed in some detail.

Ab initio bending potential-energy curves for Rydberg states of H 2 O

Using a basis set containing Rydberg s, p and d functions, ab initio configuration interaction calculations have been made for the bending potentialenergy curves of Rydberg states of H2O having excitation energies up to about 12eV. The calculations reveal a large number of avoided crossings between ‘bent’ Rydberg states arising from the excitation of an electron from the 1b1 orbital and ‘linear’ states arising from the excitation of an electron from the 3a1 orbital. Quantum defect functions have been derived from the ab initio results and used to generate bending potential-energy curves for higher Rydberg states.

Effects of subsurface Na, H and C on CH 3 adsorption on Ni(111)

Ab initio valence orbital configuration interaction calculations are used to study the energy effect of Na, H and C atom subsurface species on CH 3 chemisorption at a hollow 3-fold site on Ni(111). The lattice is modeled as an embedded three layer cluster of 41 atoms. Ni 3d orbitals are explicitly included on seven nickel atoms on the surface. The calculated chemisorption energies of pyramidal CH 3 on Ni(111) are 38 for the clean surface and 50, 47, and 17 kcal/mol for the Na, H, and C implants, respectively. The energies required to distort tetrahedral CH 3 into a planar structure are 22 kcal/mol on clean Ni(111), 30 kcal/mol with the Na implant, 24 kcal/mol with the H implant, and 12 kcal/mol with the C implant, respectively. Thus, Na below the surface may stabilize a carbon overlayer to a tetrahedral diamond structure. CH 3-surface distances, C-H stretching and surface-CH 3 vibrational frequencies are also reported.

Effects of subsurface Na, H and C on CH 3 adsorption on Ni(111)

Ab initio valence orbital configuration interaction calculations are used to study the energy effect of Na, H and C atom subsurface species on CH 3 chemisorption at a hollow 3-fold site on Ni(111). The lattice is modeled as an embedded three layer cluster of 41 atoms. Ni 3d orbitals are explicitly included on seven nickel atoms on the surface. The calculated chemisorption energies of pyramidal CH 3 on Ni(111) are 38 for the clean surface and 50, 47, and 17 kcal/mol for the Na, H, and C implants, respectively. The energies required to distort tetrahedral CH 3 into a planar structure are 22 kcal/mol on clean Ni(111), 30 kcal/mol with the Na implant, 24 kcal/mol with the H implant, and 12 kcal/mol with the C implant, respectively. Thus, Na below the surface may stabilize a carbon overlayer to a tetrahedral diamond structure. CH 3-surface distances, CH stretching and surface-CH 3 vibrational frequencies are also reported.

Isotropic and anisotropic hyperfine coupling constants of 13C and 1H for the oxirane and oxallyl radical cations. Theoretical predictions with the ab initio configuration interaction method

Ab initio single excited configuration interaction calculations of hyperfine coupling constants with Chipman's basis sets suggest that the oxirane radical cation has the ring-opened oxallyl structure, rather than a ring-closed structure.

Ab initio configuration interaction and random phase approximation calculations of the excited singlet and triplet states of uracil and cytosine

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAb initio configuration interaction and random phase approximation calculations of the excited singlet and triplet states of uracil and cytosineJames D. Petke, Gerald M. Maggiora, and Ralph E. ChristoffersenCite this: J. Phys. Chem. 1992, 96, 17, 6992–7001Publication Date (Print):August 1, 1992Publication History Published online1 May 2002Published inissue 1 August 1992https://doi.org/10.1021/j100196a027RIGHTS & PERMISSIONSArticle Views116Altmetric-Citations34LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts

Ab initio configuration interaction study of the metal-metal coupling in some zirconium(III) and titanium(III) dimers. Evidence for a superlong Zr-Zr bond in [Cp2Zr(.mu.-PR2)]2 and related complexes

The metal-metal distance in [Cp 2 Zr(μ-PH 2 )] 2 (1) and [Cp 2 Ti(r-Cl)] 2 (2) has been optimized by means of ab initio configuration interaction (CI) calculations, all other distances being kept constant at their experimental values. The computed equibrium distances are d zr-zr =3.77 A and d Ti-Ti =3.965 A in good agreement with the observed structures. The singlet-triple (S-T) splitting computed for 1 at the observed geometry is 10065 cm -1

Dissociative chemisorption of CH4 on Ni(111)

The dissociative chemisorption of methane at an atop-atom site on a (111) surface of nickel is treated using a many-electron embedding theory to describe bonding, modeling the lattice as a 41-atom, three layer cluster. Ab initio valence orbital configuration interaction (multiple parent) calculations carried out on a local surface region permit an accurate description of bonding at the surface. Ni 3d orbitals are explicitly included on seven nickel atoms on the surface. The calculated activation energy of CH4 adsorbed at an atop Ni site to produce CH3 and H coadsorbed at separated threefold sites is 17 kcal/mol. The dissociation of CH4 to CH3(ads)+H (ads) is predicted to be 2.8 kcal/mol exothermic. The Ni 3d orbitals contribute to the bonding by directly mixing with methane C–H orbitals during the dissociation process and through a direct interaction of 3d9 and 3d10 configurations at the transition state. The dissociation pathway and the bonding properties of adsorbed CH4 and coadsorbed CH3 and H are discussed.