Ab Initio Configuration Interaction Calculations Research Articles

The electronic spectrum of linear HC 9H

Large-scale ab initio multi-reference configuration interaction (MRD-CI) calculations are carried out to determine the vertical electronic spectrum of HC 9H. The calculated energy for the first dipole-allowed 3Σ u −← X 3Σ g − transition of 2.38 eV is in reasonable agreement with recent measurements of 582 nm (2.13 eV) in neon matrix. The oscillator strength is computed with f=0.007. In addition a second very strong ( f=4.7) 3Σ u −← X 3Σ g − transition is predicted by the calculations to be located near 6 eV.

Electronic states and transitions of the TeX (X = C1, Br, I) radicals

Ab initio multireference configuration interaction calculations including spin-orbit coupling have been carried out for the first time for valence electronic states of the TeX (X = Cl, Br, I) radicals and compared with the results for the isovalent TeF and IO systems obtained earlier at a similar level of theoretical treatment. The calculated spectroscopic constants are in good agreement with experimental data in the rare cases when the latter are available. It is shown that the X2 II(σ2π4π*3) ground state bonding becomes consistently weaker down the TeX group (calc. De, = 25480cm−1 for TeF, 12 100cm−1 for Tel) due to the more covalent character of bonding in the heavier radicals. The first excited state, A 4Σ− (π*→ σ*), is calculated to be bound in all systems. It is split into Ω 1/2 and 3/2 components, with regular ordering in the Franck-Condon region, opposite to that of the ground X2II state. For larger internuclear distances, the A1 4Σ− 1/2 state undergoes an avoided crossing with X2 2II1/2, which causes a shoulder in the X2 potential curve and also leads to a crossing between the A1, and A2 curves and large distinctions in their vibrational frequencies. The π* → σ* B2Σ−, C2δ, and 1 2Σ+ states are calculated to lie next in energy. They are all bound in the lightest of the TeX radicals, TeF, but successively lose their bonding character down the group. In contrast to oxygen monohalides, the 22II(σ2π3 π*4) state has a repulsive potential curve. Electric dipole transition moments and radiative lifetimes have also been calculated for the low lying bound states in all systems. Most of them are found to be quite weak. The A1,2 → X1,2 spectra are dominated by parallel contributions, with the A2 → X1 being the strongest one. The T values for this transition are quite similar and lie in the 17–30 μs range. Radiative lifetime values for the B → X1,2 transitions demonstrate very irregular behaviour for various, TeX radicals, due to strong admixture of A4Σ− character to the X1,2 states near the B2Σ− potential minimum. The A1,2 4Σ− 1/2,3/2 and B2Σ− 1/2 states of TeX (X = Cl, Br, I) still await their experimental observation.

Analysis of the magnetic coupling in binuclear complexes. I. Physics of the coupling

Accurate estimates of the magnetic coupling in binuclear complexes can be obtained from ab initio configuration interaction (CI) calculations using the difference dedicated CI technique. The present paper shows that the same technique also provides a way to analyze the various physical contributions to the coupling and performs numerical analysis of their respective roles on four binuclear complexes of Cu (d9) ions. The bare valence-only description (including direct and kinetic exchange) does not result in meaningful values. The spin-polarization phenomenon cannot be neglected, its sign and amplitude depend on the system. The two leading dynamical correlation effects have an antiferromagnetic character. The first one goes through the dynamical polarization of the environment in the ionic valence bond forms (i.e., the M+⋯M− structures). The second one is due to the double excitations involving simultaneously single excitations between the bridging ligand and the magnetic orbitals and single excitations of the environment. This dispersive effect results in an increase of the effective hopping integral between the magnetic orbitals. Moreover, it is demonstrated to be responsible for the previously observed larger metal-ligand delocalization occurring in natural orbitals with respect to the Hartree–Fock ones.

An ab initio quantum-chemical study of the blue-copper site of azurin.

The electronic structure of the blue-copper site of Pseudomonas aeruginosa azurin has been investigated by ab initio multireference determinantal configuration interaction (MRD-CI) calculations. A truncated site consisting of copper and its three equatorial ligands has been studied with emphasis on the g tensor and the nitrogen hyperfine tensors of the coordinating histidines. In the ground state the singly occupied molecular orbital (SOMO) involves a copper 3d orbital pi antibonded to the cysteine sulfur and sigma antibonded to the histidine nitrogens. A proper description of the electron-paramagnetic-resonance parameters has been achieved through the use of an effective core potential for copper up to and including the 3s electrons. Both the complete g tensor and the anisotropic hyperfine tensors at the nitrogens are essentially reproduced. Mulliken spin densities of 35 and 59% on copper and sulfur, respectively, and 2.1 and 1.7% on the respective coordinating nitrogens reflect the delocalized character of the SOMO and the inequivalence of the histidines.

Ab initio spin-orbit CI calculations of the potential curves and radiative lifetimes of low-lying states of lead monofluoride

The electronic structure of the lead monofluoride molecule is studied by means of ab initio configuration interaction (CI) calculations including the spin-orbit interaction. Potential-energy curves are generated for a large number of electronic states, of which only the X1 2Π1/2 ground and X2 2Π3/2 and A 2Σ+ excited states have been observed experimentally. Two different methods are compared for the inclusion of spin-orbit effects in the theoretical treatment, a contracted CI which employs a basis of large-scale Λ–S eigenfunctions to form a rather small matrix representation of the full relativistic Hamiltonian (two-step approach), and a more computationally laborious technique which involves solution of a secular equation of order 250 000 S2 eigenfunctions of different spin and spatial symmetry to achieve a potentially more evenly balanced description of both relativistic and electron correlation effects (one-step approach). In the present application, it is found that both methods achieve quite good agreement with measured spectroscopic constants for the X1, X2, and A states. The simpler of these methods is also employed to predict the radiative lifetimes of the latter two states. The key A 2Σ+–X 2Π transition moment in these calculations is found to vary strongly with internuclear distance and to vanish in the neighborhood of the respective equilibrium distances of both participating states. The computed lifetime for the A, v′=0 state of 16 μs overestimates the corresponding measured value by a factor of three, but those of higher vibrational states are found to decrease rather sharply with increasing v′, suggesting that only a slight displacement of the theoretical A–X transition moment curve is needed to explain the above discrepancy.

Electronic structure of Ge 2 and Ge 2− and thermodynamic properties of Ge 2 from all electron ab initio investigations and Knudsen effusion mass spectroscopic measurements

The low-lying states of the molecule Ge 2 and of the ion Ge 2 − have been investigated by all electron ab initio multi-configuration self-consistent field (CASSCF) and multi-reference configuration interaction (MRCI) calculations. The relativistic corrections for the Darwin contact term and for the mass–velocity term have been determined in perturbation calculations. Treatment of the spin-orbit coupling has been included. The ground state of Ge 2 is derived as 0 g +( 3Σ g −) with equilibrium distance 2.422 Å, and vibrational frequency 270 cm −1 . The spectroscopic constants of 15 excited states are presented. Thermal functions based on the theoretically determined molecular parameters were used to derive the thermodynamic properties of the Ge 2 molecule from new mass spectrometric equilibrium data. The literature value for the dissociation energy of Ge 2 has been re-evaluated. The recommended dissociation energy, D ∘ 0(Ge 2), and enthalpy of formation, Δ f H ∘ 298.15(Ge 2), in kJ mol −1 are 260.7±6.8 and 484.8±6.8, respectively.

A study of the C(1D)+H2→CH+H reaction: Global potential energy surface and quantum dynamics

The adiabatic global potential energy surface of the CH2 system for the first singlet state of A′ symmetry (ã 1A′) has been computed. Ab initio, multireference, single and double configuration interaction calculations have been used to characterize this state. This potential energy surface has a calculated well depth of 99.7 kcal/mol relative to the C(1D)+H2 asymptote. The surface has no barrier for the perpendicular C2v geometry, but presents a large barrier (12.35 kcal/mol) for the collinear C∞v geometry. The ab initio calculations were carried out over 1748 geometries and the resulting energies were fitted to a many body expansion. Based on this surface, we have performed the first quantum reactive scattering calculations for the C(1D)+H2(X 1Σg+)→CH(X 2Π)+H(2S) reaction and total angular momentum J=0. The hyperspherical coordinates time-independent method has been used. We note that the state-to-state reaction probabilities as a function of the collision energy show a dense resonance structure which is unusual for this type of atom+diatom reaction. We present also rotational distributions.

The Rydberg states of NO2: Vibrational autoionization of the ndσ states

Ab initio configuration interaction calculations were carried out on the potential-energy surfaces of the ground and Rydberg excited electronic states of NO2. The results show that potential-energy curves with typically Rydberg form are obtained for most of the excited states at linear geometries, similar to the ground-state potential of NO2+. At nonlinear geometries valence-Rydberg interactions complicate the potential-energy surfaces of the excited states. Quantum defect functions have been determined from the ab initio results on the 3dσ Rydberg state and vibrational autoionization widths have been calculated for excited vibrational levels of members of the ndσ series.

Ab initio MRD-CI study of excited states of chloromethanol ClCH 2OH and photofragmentation along C–O and C–Cl cleavage

Large-scale ab initio multi-reference configuration interaction calculations are carried out for ground and excited states of chloromethanol ClCH 2OH to investigate photofragmentation processes relevant to atmospheric chemistry. Five low-lying excited states ( 1 3 A″ , 1 3 A ′ , 1 1 A″ , 2 1 A ′ and 2 3 A ′ ) in the energy range between 6.8 and 8.5 eV are found to be highly repulsive for C–Cl elongation leading to CH 2 OH( X 2 A ′) and Cl( 2 P) . Photodissociation along the C–O bond leading to CH 2 Cl( X 2 B 2) and OH( X 2Π) has to overcome a small barrier of about 0.3 eV because the low-lying excited states 1 1 A″ , 1 3 A ′ and 1 3 A″ become repulsive only after the C–O bond is elongated by about 0.2 Å.

Electronic states and transitions of tellurium fluoride

Ab initio multireference configuration interaction calculations including spin–orbit coupling are carried out for valence electronic states of the TeF molecule and compared to the results for the isovalent IO system obtained earlier at a similar level of theoretical treatment [S. Roszak et al., J. Phys. Chem. A 104, 2999 (2000)]. The calculated spectroscopic constants are in good agreement with available experimental data. It is shown that the X 2Π(σ2π4π*3) ground state is much more strongly bound in TeF (calc. De=25 480 cm−1) due to the greater ionic character of bonding in this system as compared to IO. The lowest excited states of TeF are found to be A 4Σ1/2,3/2− and B 2Σ1/2− which result from the π→σ* electronic excitation. In contrast to IO, the Π2(σ2π3π*4) excited state has a repulsive potential curve and is not expected to be a factor in the low-energy spectroscopy of TeF. Particular emphasis is placed on computation of the transition moments and radiative lifetimes of the TeF electronic states. Most transitions are found to be quite weak, with the strongest of them, B 2Σ1/2−→X1 2Π3/2, characterized by a τ value of 9.5 μs. The two C 2Δ spin components, Ω=5/2 and 3/2, not yet observed experimentally, are predicted to lie ∼1800 cm−1 above B 2Σ1/2− and have partial lifetimes for transitions to the X1 and X2 states which are only slightly longer than those calculated for the B→X1,2 transitions.

Construction of reproducing kernel Hilbert space potential energy surfaces for the 1 A″ and 1 A′ states of the reaction N(2D)+H2

We present in detail the construction procedure of two high quality global potential energy surfaces that have recently been used in quasiclassical and quantum dynamics studies for the reaction N(2D)+H2→NH+H. The procedure is based on the reproducing kernel Hilbert space method to interpolate high-level multireference configuration interaction ab initio calculations using augmented polarized triple zeta basis sets. A particular molecular coordinate system has been adopted to treat the nonadditive three body interaction as well as to guarantee the triangle inequalities of three molecular bond lengths and permutation symmetry of the two hydrogen atoms. A new radial reproducing kernel that remains finite at short distances while decaying to zero at large distances has been introduced, especially, for treating the molecule in the linear H–N–H configurations. Moreover, a robust procedure has been devised to handle the cusp in the two-body H2 interaction associated with the 1 A″ potential energy surface. The resulting surfaces are smooth, accurate, efficient to evaluate, exactly reproducing the input data upon which they are based and represent a significant improvement over previous surfaces for this reaction.

Valence excitations observed in resonant soft X-ray emission spectra of K2Ni(CN)4·H2O at the Ni 2p edge

Resonant soft X-ray emission spectra have been measured at the Ni 2p edge of a planar nickel complex K 2 Ni(CN) 4 ·H 2 O and interpreted by performing ab initio configuration interaction calculations. This cyano complex shows characteristic metal-to-ligand charge transfer (MLCT) bands in the Ni 2p photoabsorption spectra. Resonant soft X-ray emission via the MLCT intermediate states can be interpreted as valence MLCT excitations within a one-electron picture. On the other hand, resonant soft X-ray emission via the intra-atomic Ni 2p–3d intermediate state corresponding to the white line excitation shows significant electron correlation effects.

MRD-CI study of the electronic spectrum of linear C 9

Large-scale ab initio multi-reference configuration interaction (MRD-CI) calculations are carried out to determine the vertical electronic spectrum of linear C 9. On the contrary to prior theoretical estimations based on the free electron model, the calculated energy for the 1Σ u + ← X 1Σ g + transition of 4.43 eV confirms recent measurements of 4.20 eV. The corresponding yet to be observed 1Π u ← X 1Σ g + transition is predicted to be located near 2.37 eV.

Re-analysis of the K-shell spectrum of benzene

Abstract The carbon K-shell spectrum of gaseous benzene has been recorded by inner-shell electron energy loss spectroscopy (ISEELS) under electric-dipole conditions (2 keV, small angle) with a resolution of 0.17 eV. Ab initio configuration interaction calculations performed in the frame of the equivalent core model have helped in the assignment of the experimental spectrum. They emphasize the importance of the multi-configurational approach and the role of the Rydberg orbitals in the assignment of some features. The band located at 289.09 eV is assigned to Rydberg excitations and a 3π* transition mixed with doubly excited configurations, and not to an anti-bonding (C–H) transition as suggested previously. Because of a large 3s/3d character of the (C–H)* orbital, the corresponding peak appears above the ionization threshold. The 293.8 eV band contains in-plane resonances and very likely also several doubly excited states with a 3π* contribution.

Lifetimes and dissociation pathways of the quasi-bound states of the Na⋯FH van der Waals molecule

The highly reliable potential energy function for the ground electronic state of the NaFH system based on the ab initio multi-reference configuration interaction calculations [M.S. Topaler, D.G. Truhlar, X.Y. Chang, P. Piecuch, J.C. Polanyi, J. Chem. Phys. 108 (1998) 5349; V. Špirko, P. Piecuch, O. Bludsky, J. Chem. Phys. 112 (2000) 189] is used to calculate the energies, widths, and dissociation pathways of the quasi-bound states of the Na⋯FH van der Waals molecule. Although emphasis is placed on the quasi-bound states resulting from vibrational excitation of the HF fragment in the complex, the bound states and the quasi-bound states, in which the HF fragment is not excited, are investigated as well. The study focuses on assessing the reliability of the earlier stabilization calculations [V. Špirko, P. Piecuch, O. Bludsky, J. Chem. Phys. 112 (2000) 189] by performing independent close-coupling calculations for the outgoing scattering waves of the NaFH system. The probabilities of decay of the Na⋯FH ro-vibrational resonances into various Na+HF(v,j) channels provide new insights into the dynamics of dissociation of the Na⋯FH complex.

Ab Initio Study of the Exchange Coupling in Oxalato-Bridged Cu(II) Dinuclear Complexes

The structural dependence of the coupling constant in a series of [L3Cu(μ-C2O4)CuL3]2+complexes is analyzed by means of ab initio difference-dedicated configuration interaction (DDCI2) calculations on the model (μ-oxalato)bis[triamminecopper(II)] cation, [(NH3)6Cu2(μ-C2O4)]2+, in which the nitrogen-coordinated ligands have been substituted by NH3. Two types of geometrical structures have been considered: three different C2h geometries and four crystallographic centrosymmetric geometries taken from [(Et5dien)2Cu2(μ-C2O4)](BPh4)2 and [(Et5dien)2Cu2(μ-C2O4)](PF6)2 (Et5dien = 1,1,4,7,7-pentaethyldiethylenetriamine), [(tmen,2-MeIm)2Cu2(μ-C2O4)](PF6)2 (tmen = N,N,N‘,N‘-tetramethylethylenediamine and 2-MeIm = 2-methylimidazole), and [(dien)2Cu2(μ-C2O4)](ClO4)2 (dien = diethylenetriamine). The results show that the antiferromagnetic coupling is strongly underestimated when pure DDCI2 calculations are performed, but when the CI space includes the relaxation of the oxalato-copper charge transfer, quantitative agre...

Spin–orbit effects in photodissociation of sodium iodide

Ab initio configuration interaction calculations of the electronic binding energies, spin–orbit coupling matrix elements and transition dipole moments of NaI are presented. The results are used to construct adiabatic and diabatic representations of the 0+ molecular states relevant to predissociation. The dynamics of photopredissociation is elucidated by multichannel time-dependent wave packet propagation in the diabatic representation. Specific features associated with the spatial and temporal evolution of the wave packet are ascribed to those observed in femtosecond pump–probe experiments. In particular, the rate of decay of the electronically excited NaI* complex is found to be in close agreement with time-resolved experimental studies. Partial photoabsorption cross sections for the production of iodine atoms in the ground (2P3/2) and excited (2P1/2) spin–orbit states are calculated and found to peak at excitation wavelengths of 322 and 263 nm, respectively, in accord with experimental data.

Ab initio configuration interaction study on electronically excited 4-dimethylamino-4′-cyanostilbene

Ab initio configuration interaction calculations have been performed to examine the electronic structures of both trans-4-dimethylamino-4′-cyanostilbene (DCS) and four types of perpendicularly twisted DCSs, trans-DCS is predominantly excited into the S1 state out of low-lying excited states. The S1 state is an intramolecular charge-transfer (ICT) state in which the dipole moment is about twice as large as that in S0. The excited DCS at the 4-dimethylanilino twisted conformation, which becomes S1 in polar solvents, has a very much larger dipole moment than that in S1 to trans-DCS. This means that the geometrical structure of the twisted ICT (TICT) is the 4-dimethylanilino twisted form, not the dimethylamino twisted one which is well know from the TICT structure of 4-dimethylaminobenzonitrile.

Electronic states of CF +

Ab initio multireference configuration interaction calculations have been carried out on the ground and excited electronic states of the cation CF + of singlet and triplet spin multiplicities. The results on the ground and the first excited state, a 3Π, are in good agreement with experimental and previous theoretical results. The potential energy curve of a 3Π has a deep minimum of 2.27 eV at 1.227 Å. The potential of the 1 1Π state has a very shallow local minimum (0.10 eV) at 1.307 Å. The potential energy curves of the higher excited states show mostly shallow minima at large internuclear distance, but they are repulsive at the r e of the ground state.

Accurate prediction of large antiferromagnetic interactions in high- T(c) HgBa2Ca(n-1)Cu(n)O(2n+2+delta) ( n = 2,3) superconductor parent compounds

The in-plane nearest-neighbor Heisenberg magnetic coupling constant, J, of La2CuO4, Nd2CuO4, Sr2CuO2Cl2, YBa2Cu3O6, and undoped HgBa(2)Ca(n-1)Cu(n)O(2n+2+delta) ( n = 1,2,3) is calculated from accurate ab initio configuration interaction calculations. For the first four compounds, the theoretical J values are in quantitative agreement with experiment. For the Hg-based compounds the predicted values are -135 meV ( n = 1) and approximately -160 meV ( n = 2,3), the latter being much larger than in previous cases and, for n = 3, increasing with pressure. Nevertheless, the physics governing J in all these layered cuprates appears to be the same. Moreover, calculations suggest a possible relationship between J and T(c).