Ab Initio Configuration Interaction Calculations Research Articles

Evolution of the electronic structure of lithium clusters between four and eight atoms

Absorption spectra of lithium clusters containing four to eight atoms have been measured using depletion spectroscopy. Few intense transitions are observed, always located in two predominant spectral regions, ∼480 and 680 nm. The spectra are interpreted using ab initio configuration interaction (CI) calculations, leading to a complete characterization of the excited states and a straightforward determination of the ground state geometrical structure. Intense transitions are explained by interference effects in the transition amplitude and symmetry considerations. Comparisons with semiclassical models, in which an effective mass correction is introduced, are also presented.

A theoretical study of the CuOH molecule

Ab initio configuration-interaction calculations were carried out on the copper hydroxide (CuOH) molecule in its ground and excited states. The characteristics of these states are discussed in terms of the excitation energies and the one-electron properties. Electron correlation is found to be an important factor in the energetics of this molecule.

Chemisorption of atomic H and CH x fragments on Ni(111)

The adsorption of atomic hydrogen, methylidyne, methylene and methyl on the (111) surface of nickel is treated using a many-electron embedding theory to describe bonding, 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. All adsorbed species are found to bind strongly to the Ni(111) surface at three-fold and bridge sites with adsorption energies ranging from 1.5 to 3.1 eV. Atop Ni adsorption sites have significantly higher energy. Bond lengths, angles and vibrational frequencies for adsorbates on the surface are calculated. The geometries of the adsorbed species and the nature of the bonding to the surface are discussed.

Experimental and quantum-theoretical investigation of the circular dichroism spectrum of R-methyloxirane

Methyloxirane, as a prototype for other substituted oxirane systems, was chosen for spectroscopic and theoretical study of circular dichroism. Vacuum UV absorption and CD spectra are reported, as well as ab initio multireference configuration interaction calculations for the rotatory strengths of the first three CD absorption bands. Good agreement between theory and experiment is obtained.

Ab initio rotation-vibration energies and intensities for the H 2F + molecule

In a previous publication [I. D. Petsalakis, G. Theodorakopoulos, J. S. Wright, and I. P. Hamilton, J. Chem. Phys., 92, 2440–2449 (1990)] we reported the ab initio multireference configuration interaction calculation of the three-dimensional potential energy surface of the H 2F + molecule in the ground X ̃ 1A 1 electronic state at 119 nuclear geometries spanning an energy range up to about 50 000 cm −1 above equilibrium. We fitted the 71 points within 33 000 cm −1 of equilibrium to an analytic expression and performed variational calculation of the vibrational energies in Jacobi coordinates using the Discrete Variable Representation and Distributed Gaussian Basis functions (DVR-DGB) technique. In the present paper we examine the effect on the vibrational energies of using a surface obtained by fitting through 52 points within 25 000 cm −1 of equilibrium. We use this surface in a variational calculation of the J = 0, 1, and 2 rotation-vibration energies using the Morse Oscillator Rigid Bender Internal Dynamics Hamiltonian [P. Jensen, J. Mol. Spectrosc., 128, 478–501 (1988); J. Chem. Soc. Faraday Trans. 2, 84, 1315–1340 (1988)]. The vibrational energies obtained are compared with those obtained by the DVR-DGB technique. We also calculate ab initio the dipole moment function and rotation-vibration intensities, and we simulate the ν 2 band, which has not yet been observed.

ChemInform Abstract: Ab initio Configuration Interaction and Random Phase Approximation Calculations of the Excited Singlet and Triplet States of Adenine and Guanine

ChemInform Abstract: Ab initio Configuration Interaction and Random Phase Approximation Calculations of the Excited Singlet and Triplet States of Adenine and Guanine

The electronic states of the azines. III. Pyrimidine, studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration calculations

The electronic states of pyrimidine have been investigated by VUV absorption and near-threshold electron energy-loss spectroscopy, together with ab initio multi-reference configuration interaction calculations. This approach has allowed the experimental assignment of all the low-lying valence-excited nπ* and ππ* singlet states. In contrast to ππ* excitations, some of which are difficult to compute within 0.5 eV, the nπ* states are more accurately calculated, as are the lowest Rydberg excited states. The experimental identification of many low-lying Rydberg states is not yet possible. The UV-photoelectron spectral assignments by large-scale CI are similar to earlier Green's function results, except for the inner π level (1b 1) ionisation; many shake-up states are computed below 17 eV and it is clear that the TDA method, with the same basis set, leads to a much later onset.

Cluster calculations on localized holes in La 2CuO 4

The electronic structure of [CuO 2] 2− sheets, as occur in La 2CuO 4, is studied by ab initio non-orthogonal configuration interaction (NOCI) calculations on the small copper-oxide clusters CuO 4 and Cu 2O 7. In the NOCI approach the wave function is obtained from CI calculations between relaxed Hartree-Fock states. With this approach important local relaxation effects, as well as charge transfer interactions can be taken into account. The cluster ground states can be characterized by Cu(d 9) O(p 6). States with one extrinsic hole have predominant Cu(d 9) O(p 5) character with admixture of Cu(d 8) O(p 6). This admixture is especially important for states with the extrinsic hole mainly in O( p σ ).

Ab initio configuration interaction and random phase approximation calculations of the excited singlet and triplet states of adenine and guanine

Ab initio configuration interaction and random phase approximation calculations of the excited singlet and triplet states of adenine and guanine

Anab initiocalculation of the rotational-vibrational energies in the electronic ground state of NH2

We have calculated ab initio the three-dimensional potential-energy surface of the NH2 molecule at 145 nuclear geometries spanning energy ranges of about 18 000 cm-1 for the NH stretch and 12 000 cm-1 for the bend. The ab initio configuration-interaction calculations were done using the multireference MRD-CI method. The calculated equilibrium configuration has NH bond length r e = 1·0207 A and bond angle α = 103·1°. The rotational-vibrational energies for 14NH2, 14NHD and 14ND2 were calculated variationally using the Morse-oscillator rigid-bender internal-dynamics Hamiltonian. For 14NH2 we calculate that υ1 = 3267 (3219) cm-1, υ2 = 1462 (1497) cm-1 and υ3 = 3283 (3301) cm-1, where experimental values are given in parentheses.

A theoretical study on the mechanism of charge transfer state formation of 4-(N,N-dimethylamino)benzonitrile in an aqueous solution

The mechanism of charge transfer (CT) state formation of excited state 4-(N,N-dimethylamino)benzonitrile (DMABN) in an aqueous solution has been studied theoretically. Ab initio configuration interaction (CI) calculations were carried out for the potential energy surfaces of ground and excited state DMABN. The potential surface of second excited S2 state was represented by a superposition of three diabatic states, one is of the ion pair type and the other two of the neutral ones, to facilitate the calculations in a polar solution. The intermolecular pair potentials between DMABN and H2O were developed with the aid of electron distributions in DMABN obtained from ab initio calculations. These potential functions were applied to determine the geometries of DMABN–H2O complex and the results were compared with the available experimental data. Monte Carlo simulation calculations were further performed for the aqueous solution of DMABN. The potentials of mean force for the torsional angle of dimethylamino group revealed that the S2 state potential profile is remarkably altered due to the solvation and the twisted intramolecular CT state becomes a stable point on the surface while this point corresponds to the top of potential barrier in the gas phase. The origin of broad emission band at a longer wavelength region observed in the experiments were discussed on the basis of present calculations. In order to elucidate the mechanism of CT state formation, the reaction free energy surfaces were constructed as the function of solvation coordinate and amino torsional angle. The results obtained here were that: (a) the shape of free energy curve of S2 state is far from a parabolic form along the solvation coordinate while the S1 state curve is nearly parabolic; and (b) the torsional coordinate is required to undergo a deformation to reach the transition state region of CT state formation reaction.

Electronic states and nature of bonding of the molecule NiSi by all electron ab initio HF-CI and CASSCF calculations

All electron ab initio Hartree-Fock (HF), configuration interaction (CI) and multiconfiguration self-consistent field (CASSCF) calculations have been applied to investigate the low-lying electronic states of the NiSi molecule. The ground state of the NiSi molecule is predicted to be1Σ+. The chemical bond in the1Σ+ ground state is a double bond composed of one σ and one π bond. The σ bond is due to a delocalized molecular orbital formed by combining the Ni 4s and the Si 3pσ orbitals. The π bond is a partly delocalized valence bond, originating from the coupling of the 3dπ hole on Ni with the 3pπ electron on Si. Withing the energy range 1 eV 18 electronic states have been identified. The lowest lying electronic states have been characterized as having a hole in either the 3dπ or the 3dδ orbital of Ni, and the respective final states are formed when either of these holes are coupled to the 3pπ valence electron of Si.

The electronic states of the azines. II. Pyridine, studied by VUV absorption, near-threshold electron energy loss spectroscopy and ab initio multi-reference configuration interaction calculations

The electronic states of pyridine have been investigated experimentally, using VUV absorption and near-threshold electron energy loss spectroscopy, and theoretically, with ab initio multi-reference configuration interaction calculations. The theoretical energies relate well to the experimental observations, allowing firm identification of all low-lying ππ* and nπ* excited states, including an optically forbidden nπ* 1A 2 state, detected at 5.43 eV in electron energy loss. Fine structure in the VUV spectrum, associated with Rydberg-state excitation, matches that observed in high resolution photoelectron spectra, facilitating Rydberg-state assignments. Good energies are calculated for n = 3, 4 members of Rydberg series. The Cl and MC SCF results confirm that the first ionisation potential in pyridine is 2A 1 corresponding to removal of a non-bonding electron, in agreement with experiment. A triple zeta valence plus polarisation SCF + all-valence electron Cl has led to good agreement for various one-electron operators of the wavefunctions with experiment.

Molecular excited states versus collective electronic oscillations: Optical absorption proves of Na 4 and Na 8

Photodepletion spectra were obtained for Na 4 and Na 8 over the wavelength range from 435 to 825 nm. For Na 4, six band systems were found with cross sections from 0.25 to 3.39 Å 2. They are assigned to various electronic transitions from rhombic ground state Na 4 predicted in ab initio large-scale configuration interaction calculations. For Na 8, we confirm the observation of a strong transition centered at 490 nm which has been interpreted in terms of a classical surface-plasma oscillation. The enhanced signal-to-noise available to us allows the first resolution of fine structure incompatible with this classical picture.

Electronic and vibrational transitions observed in collisions between C +2 and He

Electronic and vibrational transitions among the low-lying doublet and quartet states of C + 2 ions are investigated by means of high resolution collision spectroscopy. The presented translational energy spectra of 3 keV C + 2 ions scattered off a He target clearly show a number of excitation/de-excitation transitions, the lowest of which is centred at ∼ ±0.35 eV. Assignment of this transition to C + 2 ( X 4Σ − g, ν = 2 ↔ ν = 0) is based on the existing ab initio configuration interaction (CI) calculations. The observation of this and other similar transitions is the first experimental evidence which shows that vibrationally excited C + 2 ( X 4Σ − g) ions formed by electron impact can live for at least a few microseconds (ion transit time between the ion source and the target cell).

Ab initio MRD CI calculations on the cesium hydride (CsH) molecule

Ab initio multi-reference configuration interaction (MRD CI) calculations were carried out for the potential energy curves of the first 17 electronic states of the CsH molecule up to large bond distances (20 bohr). The1Σ+ states were also calculated by means of relativistic all-electron SCF and CI using the spin-free no-pair operator with external field projectors. For the low-lying states, the spectroscopic parameters were determined. Dipole moments as well as the transition dipole moments: μ(X1 Σ+ →A1 Σ+), μ(X1 Σ+ →B1 Σ+), μ(A1 Σ+ →B1 Σ+), were also calculated. Non-relativistic and relativistic results are compared. An analysis of the interactions in the1,3Σ+ states is also proposed.

The adsorption of water and hydroxyl on Ni(lll)

The adsorption of water and hydroxyl on the (111) surface of Ni is treated using a many-electron embedding theory to describe the electronic bonding, 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. Molecular H 2O adsorbed on the Ni(111) surface is found to prefer an atop atom site with an adsorption energy of 12 kcal/mol and a Ni-O equilibrium distance of 2.06 Å. The equilibrium geometry of H 2O is calculated to lie in a plane inclined by about 25° to the normal to the surface, but tilting the plane of the molecule from 0° to 50° or rotating the molecule about the Ni-O axis changes the energy only slightly. The OH radical binds strongly to the Ni(111) surface at both three-fold and bridge sites with adsorption energies of 87 kcal/mol and Ni-O bond lengths from 2.02–2.08 Å. The OH axis of adsorbed OH is inclined about 10° from the surface normal at a three-fold site. Dissociation of H 2O to OH and H adsorbed at nearby three-fold sites is exothermic, and for OH and H at a large distance of separation, the reaction H 2O(ads) α OH(ads) + H(ads) is 52 kcal/mol exothermic. A high energy barrier is found at the initial stage of dissociation. The work function decreases by ∽0.5 eV on H 2O adsorption and increases by ~0.2 eV on OH adsorption.

Structure and stability of Mg+n and Mg++n clusters (n≤7)

We present a nonempirical model Hamiltonian of the valence bond type for singly and doubly charged clusters of atoms which have a s2(1S) ground state configuration. This model includes delocalization, instantaneous polarization energies, interaction between induced dipoles, and Coulombic repulsion of the holes in doubly charged clusters; up to four-body terms are included. The model is applied to magnesium clusters. Comparisons with ab initio (configuration interaction) calculations show overall agreement for singly charged clusters, whereas doubly charged clusters suffer from the lack of collective p-type contributions in model calculations. For singly charged magnesium clusters, ground state geometries are found to be neutral atoms surrounding either a charged atom or a charged dimer. Considerations of shape and energy show that clusters are easily obtained by aggregation processes and analysis of vertical ionization potentials shows that they can also be obtained by direct ionization of neutral species.

Low-lying negative-ion states of calcium.

Studies of electron transmission through Ca vapor reveal two prominent resonances in the range 0--4 eV. The energies and widths of these temporary anion states suggest that they may be assigned to the strongly mixed (4${s}^{2}$3d${)}^{2}$D and (4s4${p}^{2}$${)}^{2}$D states of ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$. This interpretation is supported by ab initio configuration-interaction calculations and is consistent with previously unassigned structure observed in the photodetachment cross section.

Vibrational dynamics of hydrogen bonds: The system OH −·H 2O

A complete vibration-(internal rotation) Hamiltonian for the hydrogen bonded ionic system OH −·H 2O is developed following the general idea of the Hougen-Bunker-Johns approach [J. T. Hougen, P. R. Bunker, and W. W. C. Johns, J. Mol. Spectrosc. 34, 136–172 (1970)]. In this formulation the large-amplitude antisymmetric O⋯H⋯O stretching mode (hydrogen tunneling motion) and the internal rotation are removed from the vibrational problem by defining a molecular reference configuration as a function of the antisymmetric stretching and the internal rotation coordinates. Ab initio configuration interaction calculations are performed to investigate the molecular structure of the H 3O 2 − ion and to determine certain sections of the multi-dimensional potential energy hypersurface. An analytic potential function is obtained by fitting to the ab initio energy points and this function is used to calculate symmetric and antisymmetric O⋯H⋯O stretching and internal rotation energy levels within the limitations of an extended adiabatic approximation. In addition, using a presently calculated electric dipole moment function and the approximate vibrational eigenfunctions, dipole transition moments are evaluated for the symmetric and antisymmetric O⋯H⋯O stretching states.