Ab Initio Configuration Interaction Calculations Research Articles

Inner-shell d excitation in gas phase heavy-metal halides by synchrotron radiation photoemission and ab initio CI calculations

Sn 4d electron excitations have been investigated in SnBr 2 and SnCl 2 free molecules experimentally and theoretically. The total ion yield (TIY) spectrum of tin dibromide in the Sn 4d excitation region (24 – 35 eV) is reported. The assignment of the experimental spectrum of SnBr 2 is based on the excitation spectrum calculated at ab initio Configuration Interaction (CI) level, as well as on the comparison with experimental and theoretical data previously obtained for SnCl 2. Photoion efficiency (PIE) curves obtained in the same photon energy region show that some photofragment ion channels are preferentially involved in the decay of these resonances in both SnBr 2 and SnCl 2.

The electronic structure and luminescence properties of porous silicon and silicon nanoclusters

We present the results of ab initio Hartree-Fock and configuration interaction calculations performed on selected small silicon clusters with a view to study their luminescent properties. We found that the covalent character of bonding is much reduced at the surface of the silicon clusters. The charge density of the clusters considered tends to be polarized towards their bulk-like core. This reflects the ‘self-passivation’ character of the chemical bonding in clusters. The optical transitions in the Si 18 cluster are found to have essentially many-electron character. The energies of three lowest optical transitions are found to be in the near-IR region and the corresponding oscillator strengths are 0.0001–0.004. Our results suggest that there is a strong dependence of the optical properties of silicon nanostructures not only on their size but also on their shape, which strongly affects the charge density and the character of correlation between the excited electrons.

Electronic Structure of Hf@C28 and Its Ions. 2. CI Calculations

Ab initio configuration interaction (CI) calculations were performed on the ground and excited state of Hf@C{sub 28} and its positive and negative ions. Relative core potentials, spin-orbit operators, and symmetry-adapted functions were included in the calculations to take into account the relativistic effects of the heavy atom and to reduce the time and effort of the computation. Double-zeta basis sets were used. Single and double excitations were included in the CI calculations. A number of excitation energies were obtained for Hf@C{sub 28} and its ions. The ground states of Hf@C{sub 28}{sup +}, Hf@C{sub 28}, and Hf@C{sub 28}{sup -} in T{sub d} symmetry were found to be {sup 2}A{sub 1}, {sup 1}A{sub 1}, and {sup 2}E. For Hf@C{sub 28}, values found were the following: ionization potential, 8.08 eV; electron affinity, 0.66 eV; and first excitation energy (to {sup 3}E state), 5. 26 eV. The binding energy for Hf and C{sub 28} to form Hf@C{sub 28} was obtained as 0.71 eV. These results were compared with those from our previous SCF calculations and with experimental data or other theoretical values where available. 27 refs., 2 figs., 7 tabs.

Experimental and theoretical study of the B–Ne nonbonding interaction: The free-boundB 2Σ+–X 2Π electronic transition

We report a new investigation of the interaction between atomic boron, in both the ground 2p 2P and excited 3s 2S electronic states, with Ne. BNe complexes are formed in a pulsed free jet expansion and detected by laser fluorescence excitation. A broad, asymmetric feature is seen, with maximum intensity ∼270 cm−1 to the blue of the 3s 2S–2p 2PJ atomic transition. This feature corresponds to electronic excitation from the ground vibrational level of the BNe(X 2Π1/2) state into the BNe(B 2Σ+) state, which is unbound. High level ab initio configuration–interaction calculations, involving large atomic orbital bases, were carried out to describe the relevant potentials. The potential curve for the B state reveals a broad shoulder, but no well. The calculated potential curves are corrected, very slightly, by an additional scaling of the correlation energy. Spectral simulations based on these corrected curves reproduce, nearly quantitatively, the experimental spectrum.

Long-lived, doubly charged diatomic and triatomic molecular ions

Lifetimes of doubly charged diatomic and triatomic molecules have been measured by monitoring the decay curves of such ions in a heavy-ion storage ring. CO2+, N22+, CO22+, CS22+ and SH2+ are all found to possess long-lived components which survive for time periods greater than a few seconds. All these dications are found to be essentially stable and their ultimate destruction is due to interactions with residual gases in the ring. CO2+ possesses many more lifetime components in the millisecond range than the isoelectronic N22+ ion. Translational energy spectrometry experiments on the latter species also fail to reveal any short-lived (microsecond) components. Ab initio configuration interaction calculations have been carried out and the potential energy curve for the lowest-energy metastable state of N22+ (1 Sigma g) has been determined, along with Franck-Condon factors for vertical transitions to different vibrational levels from the ground state of neutral N2; tunnelling times of each vibrational level have been computed.

Ab initio chemisorption studies of H on Fe(110)

Ab initio configuration interaction calculations are performed to study the chemisorption of atomic H on a Fe(110) surface. The lattice is modeled as an embedded three-layer, 40-atom cluster with the Fe atoms fixed at the bulk position. Fe 3d orbitals are explicitly included on five Fe atoms on the surface. Hydrogen strongly binds to the Fe(110) surface at the long-bridge, short-bridge, and quasi three-fold sites. The calculated adsorption energies are 2.76, 2.73, and 2.71 eV, respectively. H-surface bonding at the on-top Fe site is more than 0.4 eV weaker. The calculated H-surface distances are 0.89, 1.03, and 0.87Åfor H at the long-bridge, short-bridge, and quasi three-fold sites, respectively, which agrees well with the LEED value of 0.9 ± 0.1Å. The H-surface stretching vibrational frequencies are calculated to be 1070, 1066, and 1073 cm −1, at the long-bridge, short-bridge, and quasi three-fold sites, respectively. The work function of Fe(110) decreases on H adsorption. The present calculations indicate that H diffusion into the bulk through the short-bridge site will have a much higher activation barrier than via the long-bridge and quasi three-fold sites.

Potential curves and nonadiabatic matrix elements for collisions involving fragments of the HeN+ molecular ion

Ab initio multireference CI calculations have been carried out for the HeN+ molecular ion in order to describe collision processes between its constituent neutral and ionized atoms. The accuracy of these calculations is evaluated by means of a comparison of results obtained at large internuclear separations with the corresponding asymptotic energies deduced from atomic spectral data. Energy values are computed for the eleven lowest He++N and He+N+ atomic limits and average discrepancies relative to the experimental excitation energies up to 110 000 cm−1 are found to lie in the 1000–3000 cm−1 range, of which only 200 cm−1 appears to be the fault of the configuration interaction (CI) technique itself, with the main portion of the error stemming from the choice of atomic orbital (AO) basis instead. The HeN+ X 3Σ− ground state is calculated to have a De value of only 1414 cm−1, but the excited 2 3Π state has a much larger value of 22 133 cm−1 by virtue of an avoided crossing with the lower state of this symmetry. The corresponding radial nonadiabatic coupling is responsible for a large cross section for an excitation process between the N+(3Pg)+He and N+(3Du)+He channels which indirectly provides an efficient electron-capture mechanism leading to the N(4Su)+He+ exit channel. Additional nonadiabatic matrix elements for rotational and spin–orbit coupling have also been obtained and analyzed, as well as transition moments between the various HeN+ molecular states calculated.

Ab initio configuration interaction study of the electronic spectrum of SiH +

Ab initio configurationi nteraction calculations have been carried out for SiH + using a large Gaussian basis set. Calculated spectroscopic constants of the X 1Σ + and A 1Π states as well as the excitation energy and the tf 00 value of the A 1Π → X 1Σ + transition and the lifetime τ 0 of the A 1Π state are obtained in good agreement with experiment. In addition, the energies of the various asymptotes (up to 13 eV above the X 1Σ + minimum) computed for large internuclear separations are found to agree with observed atomic values to within 0.3 eV in each case. Spectroscopic constants, excitation energy, lifetime and other quantities are predicted for a large number of higher excited states. Many other bound states have been indicated in the present calculations and their computed spectroscopic constants should provide a basis for further experimental investigations of the spectrum of this ion. Special emphasis is placed on the long-range characteristics of the SiH + potential curves on the way to their respective atomic asymptotes.

A modified Hückel calculation on the structures and ionization potentials of Ag clusters

AbstractThe geometrical structures and ionization potentials of silver clusters are studied by a modified Hückel theory including the variable Coulomb integral resulting from different coordinate environments. The stable atomic configurations of the Agn clusters are obtained by comparing the energy of different structural isomers. The clusters with magic number atoms are found to adopt the near spherical host‐like structure with higher symmetry, corresponding to the complete electronic shell. Based on those favorable structures the size dependent ionization potentials of Agn are calculated up to 68 atoms by using this new developed modified Hückel theory. The theoretical values are in quite agreement with the experimental results as well as ab initio configuration interaction (CI) calculations. A transition from “electronic order” to “atomic order” around n = 40 is proposed.

The fragmentation of SH(A2?+): Ab initio calculations of spin-orbit and coriolis interactions

The electronic structure aspects of the fragmentation of the A2Σ+ state of SH via the predissociation and photodissociation processes, SH(A2Σ+, v, N, F1/F2) S(3PJ) + H(2S) and SH(X2Π, v, N, e/f) SH(A2Σ+, v, N, F1/F2) S(3PJ) + H(2S) are considered by ab initio configuration interaction (CI) calculations. All the relevant interstates A2Σ+, X2Π, 14Σ−, 12Σ−, and 14Π spin-orbit induced matrix elements, treated within the full microscopic Breit-Pauli approximation, and Coriolis (electronic orbital angular momentum) couplings are determined based on wavefunctions comprised of 680000-860000 configuration state functions. © 1995 John Wiley & Sons, Inc.

Theoretical potential-energy functions and spectroscopic data for the PH2 + ion

Three-dimensional potential-energy functions for the X 1A1 and Ã1B1 states of PH2+ have been derived from extensive ab initio multi-reference configuration interaction (MRCl) calculations. Spectroscopic constants and quartic force fields have been derived from the potential-energy functions by second-order perturbation theory. These data are believed to be reasonable predictions for this molecular ion for which there is little experimental data. Rovibrational levels have been calculated by a variational method up to 10 000 cm–1 for the ground state and up to 8000 cm–1 for the excited state. Coriolis coupling and Fermi and Darling–Dennison resonances have been investigated in detail.

Experimental and theoretical studies of the (C 1s−1,π*)3Π state of CO: Momentum transfer dependence and vibrational structure

The intensity of the X 1Σ+→(C 1s−1,π*)3Π transition of CO has been measured by electron energy loss spectroscopy using a range of scattering angles (0°–45°) and impact energies (376 to 1806 eV) in order to investigate the momentum transfer dependence of a spin forbidden inner-shell excitation. A Franck–Condon factor analysis of the vibrational structure of the singlet and triplet (C 1s−1,π*) states was used to quantify differences in the potential energy curves of these states. Ab initio self-consistent field configuration interaction (SCF-CI) calculations were carried out to generate the potential curves of the 1Π and 3Π(C 1s−1,π*) states. The electronic and vibrational energies and Franck–Condon factors are in good agreement with the experimental results. The calculations indicate that the difference in the 1Π and 3Π potential curves are related to differences in relaxation of both the (active) π* and other (passive) valence electrons.

The vacuum-ultraviolet photodissociation of NH2(X̃ 2B1)→NH(A 3Π)+H

The photodissociation of NH2→NH(A 3Π)+H was investigated by photolyzing NH2 in a flow system with tunable synchrotron radiation from 200 to 105 nm and other vuv light sources. The NH photofragments were analyzed by their triplet emission at 336 nm. Additionally, ab initio configuration interaction calculations were performed for the electronic states of NH2 involved in the photodissociation process. Vertical excitation energies, bending potentials for the excited states, Franck–Condon factors, and transition moments were calculated in order to interpret the experimental observations. The following picture evolves for the dynamics of the NH2 photodissociation: At about 7.8 eV, NH2 is excited to the 2 2A1(A′) state, which possesses the same bending angle as the X̃ 2B1 ground state. The upper state correlates with the fragments NH(A 3Π)+H. Since the bending angle is not changed, the NH(A) radicals are formed with little rotational excitation. However, the symmetric stretch becomes excited at the beginning of the dissociation leaving the NH(A) fragment with vibrational excitation. Because of symmetry conservation, the formation of the Π(A′) component of NH(A) is preferred. In the region of ∼9 eV, transitions to the 1 2A2 and/or 3 2B1 states (both have A″ symmetry in Cs) occur. The bending potentials of both states have minima for linear configurations. Therefore, the structure of the excitation spectrum is determined by a progression in the bending motion and a preferred population of high rotational NH(A) levels is observed. Vibrational excitation is small suggesting that the unbroken NH bond stays unchanged during the dissociation process. According to symmetry conservation, the Π(A″) component of NH(A) is preferably formed.

Theoretical study of the low-lying states of GeC

Energies and structural properties of the valence electronic states of the as yet unknown GeC molecule have been determined via ab initio multireference configuration interaction calculations. The ground state is found to have a 3Π symmetry. A simple discussion relating to the nature of the ground state of group IV-A diatomics is given on the basis of a competition between orbital energies and Coulombic effects.

Adsorption of CH 3O on Ni(111)

The adsorption of methoxy on Ni(111) is treated using a many-electron embedding theory, modelling the lattice as a 28-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 four Ni atoms in the local surface region. Methoxy binds strongly to the Ni(111) surface at both three-fold and bridge sites with adsorption energies of 90 and 87 kcal/mol and O-surface distances of 1.51 and 1.59 Å, respectively. The calculated adsorption energy for methoxy at an atop Ni site is 61 kcal/mol with an O-surface distance of 1.82 Å. The methoxy CO axis is normal to the surface or inclined 5° or 10° from the surface normal at three-fold sites and is inclined 20° from the surface normal at bridge sites. For methoxy adsorbed at three-fold and bridge sites, the calculated CO stretching frequencies are 1025 and 1010 cm −1, and the CH 3O-surface perpendicular stretching frequencies are 495 and 380 cm −1 respectively. The 5a 1, 1e and 2e orbitals of adsorbed methoxy are the orbitals principally involved in bonding with the surface. The bonding of methoxy to the nickel surface is characterized as a combination of ionic and covalent bonding with substantial mixing with Ni 3d orbitals. Adsorption studies of CH 3ONi and CH 3ONi 3 are also reported. Methoxy basis effects and bonding properties of methoxy upon adsorption on Ni are discussed.

Ab initio investigation of the circular dichroism spectra of methylthiirane and trans-2,3-dimethylthiirane

Ab initio multireference configuration interaction calculations were performed for R-methylthiirane and 2,3-(R,R)-dimethylthiirane in order to determine and elucidate their circular dichroism spectra based on the calculated rotatory strengths and excitation energies. The results enable the assignment of the experimental spectra (Breest et al., Mol. Phys., in press) up to an energy of 7.5 eV.

Photofragment spectroscopy and dissociation dynamics of N2+2: A 1Πu–X 1Σ+g

The 9–0 and 9–1 transitions of the A 1Πu–X 1Σ+g system of N2+2 have been observed by fast ion beam photofragment spectroscopy. Multireference configuration interaction ab initio calculations predict v′=8 to be the highest bound level which seemed to be supported by experimental evidence. The dominant predissociation mechanism for the v′=9 level appears to be through tunneling, unlike the v′=6, 7, and 8 levels, which show an e/f symmetry-dependent electronic predissociation. Calculated tunneling lifetimes from our own modified A 1Πu state potential gives good agreement with the experimental lifetimes. In addition to the 9–1 and 9–0 bands, we have also measured the previously unobserved 7–0 and 8–0 rovibronic transitions thus extending Cosby, Müller, and Helm’s study to the blue [Phys. Rev. A 28, 766 (1983)]. Molecular constants have been determined for the v=0 of the X 1Σ+g state and v=7, 8, and 9 of the A 1Πu state.

Relativistic Effect on Multiplet Terms of Rare Earth Ions

Ab initio Spin-Orbit Configuration Interaction (SOCI) calculations for the trivalent lanthanide group ions are presented for the special purpose to investigate the relativistic SO effects on their multiplet terms. The effective nuclear charges (Z eff 's) for one-body spin-orbit Hamiltonian are calculated by an atomic Dirac-Slater Xα equation and applied to the lanthanide ions. The relativistic effects of core electrons can easily be included in the reduction of Z eff and the multiplet levels shift up to 200 cm -1 by the reduction. The multiplet energies obtained by the present method are in good agreement with experimental values.

Theoretical study of the circular dichroism and VUV spectra of trans-2,3-dimethyloxirane

Ab initio configuration interaction calculations with various molecular orbital basis sets have been carried out for the first states of each type (X 1A, ten n(b)→Ryd, n(b)→σ*, four σ→Ryd, σ→σ*) of trans-2,3-dimethyloxirane in order to elucidate its circular dichroism and VUV spectra. Particular attention was placed on finding the energetic location of the first valence states. Localized molecular orbitals based on the 2B cation are found to give the best agreement and allow for definite assignment of the spectra up to ≈ 8.5 eV. The CD signals (rotatory strengths) of dimethyloxirane could also be explained based on its electronic structure.

Theoretical potential-energy functions and the rovibronic spectrum of the SiH + 2 Ion

Three-dimensional potential-energy functions for the X2A1 and A2B1 states of SiH+2 have been derived from extensive ab initio multi-reference configuration interaction (MRCI) calculations. Spectroscopic constants have been derived from the potential-energy functions by second-order perturbation theory. The calculated values are in very good agreement with the experimental data and also provide a secure value for the Ao rotational constant for the X2A1 state. Rovibronic levels were calculated by a variational method which takes into account Renner–Teller coupling. Theoretically derived transition wavenumbers are in good agreement with experimental results.