Configuration Interaction Study Research Articles

Relativistic configuration interaction study of the electronic spectrum of SnTe and SnTe+

Ab initio based relativistic configuration interaction calculations have been performed to study the electronic spectrum of the heaviest tin chalcogenide and its monopositive ion. Potential energy curves and spectroscopic constants of low-lying states of both species within 7 eV are reported. The ground-state dissociation energies of SnTe and SnTe+ are computed to be 3.48 and 2.50 eV, respectively. The spin-orbit splitting between the two components of the X 2Pi state of SnTe+ is about 3030 cm(-1). Effects of the strong spin-orbit coupling on the potential curves and spectroscopic properties of both the species are investigated in detail. The electric dipole moments of some of the low-lying states of SnTe and SnTe+ are reported. Transition moments of some important spin-allowed and spin-forbidden transitions are calculated from the configuration interaction wave functions. The radiative lifetime of the excited E 1sigma0+(+) state of SnTe is about 39 ns. The X2-X1 transition in SnTe+ is found to be more probable than the similar transition in the lighter ions. The vertical ionization energy of SnTe in the ground state is estimated to be 8.22 eV.

Electronic spectra and photodissociation of vinyl chloride: A symmetry-adapted cluster configuration interaction study

The vertical absorption spectrum and photodissociation mechanism of vinyl chloride (VC) were studied by using symmetry-adapted cluster configuration interaction theory. The important vertical pi --> pi* excitation was intensively examined with various basis sets up to aug-cc-pVTZ augmented with appropriate Rydberg functions. The excitation energy for pi --> pi* transition obtained in the present study, 6.96 eV, agrees well with the experimental value, 6.7-6.9 eV. Calculated excitation energies along with the oscillator strengths clarify that the main excitation in VC is the pi --> pi* excitation. Contrary to the earlier theoretical reports, the results obtained here support that the C-Cl bond dissociation takes place through the n(Cl-)sigma(C-Cl)* state.

Ab Initio multireference configuration-interaction study of hydrogen molecule activation by Cs-promoted Pt clusters

The adsorption of the H2 molecule on CsnPt(5-n) bcc (111) clusters for Cs/Pt rates of 20%, 40%, and 80% is studied using ab initio multiconfigurational self-consistent field plus multireference configuration-interaction variational and perturbative calculations. The H2 interaction with the clusters is studied in ground and excited states with geometry optimization, where the hydrogen adsorption takes place by a Pt atom. These calculations are compared with those of H2 adsorption on Pt4. The most stable configurations of Cs/Pt4 and Cs2Pt3 clusters (Cs/Pt rates of 20% and 40%) are a doublet and a closed-shell singlet, respectively. Both clusters capture and activate the hydrogen molecule and their behaviors resemble Pt4. The H2 capture distances are, respectively, similar and smaller than Pt4 capture distances, while the H-H bond dissociation distances are similar and bigger than those of Pt4; however, none of them presents activation barriers. The most stable Cs4Pt cluster (Cs/Pt rate of 80%) is also a closed-shell singlet; it also captures and activates the hydrogen molecule and shows a different behavior as compared with Cs/Pt4, Cs2Pt3, and Pt4 clusters. The capture distance is quite smaller and is obtained after surmounting an activation barrier. For all clusters studied here, no hydrogen absorption was observed, only the adsorption of H2.

Ab initio configuration interaction study on the energetics and electronic structure of the 1–5 2Σ + and 1–3 2Π states of CS +

The energetics and electronic structure of the 1–5 2Σ + and 1–3 2Π states of CS + at and around the equilibrium internuclear distance R e for the CS X 1Σ + state are studied by carrying out ab initio configuration interaction (CI) calculations. The spectroscopic constants of T e, ω e, and R e for the 1–4 2Σ +, 1 2Π, and 3 2Π states are evaluated from the CI potential energy curves (PECs). The avoided crossing between the 2–3 2Σ + PECs causes the 3 2Σ + minimum and explains the observed high intensities for the photoionization from the CS X 1Σ + state to both the 2–3 2Σ + states. The avoided crossing between the 3–4 2Σ + PECs produces the 3 2Σ + maximum and 4 2Σ + well minimum. The avoided crossing between the 2–3 2Π PECs results in the 3 2Π minimum and a small minimum spacing (0.14 eV) between the PECs.

Ab initio configuration-interaction study of the ground and low-lying electronic states of NiI

For the first time, we have studied the potential-energy curves, spectroscopic terms, vibrational levels, and the spectroscopic constants of the ground and low-lying excited states of NiI by employing the complete active space self-consistent-field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified six low-lying electronic states of NiI with doublet spin multiplicities, including three states of Delta symmetry and three states of Pi symmetry of the molecule within 15 000 cm(-1). The lowest (2)Delta state is identified as the ground state of NiI, and the lowest (2)Pi state is found at 2174.56 cm(-1) above it. These results fully support the previous conclusion of the observed spectra although our computational energy separation of the two states is obviously larger than that of the experimental values. The present calculations show that the low-lying excited states [13.9] (2)Pi and [14.6] (2)Delta are 3 (2)Pi and 3 (2)Delta electronic states of NiI, respectively. Our computed spectroscopic terms, vibrational levels, and spectroscopic constants for them are in good agreement with the experimental data available at present. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally.

Configuration interaction study of the electronic spectrum of SnSe +

The electronic spectrum of SnSe + has been studied from the ab initio based multireference configuration interaction calculations which include relativistic effective core potentials of both the atoms. Potential energy curves of all 12 low-lying states, which dissociate into the lowest asymptote Sn +( 2P u) + Se( 3P g), have been constructed. The ground-state dissociation energy is estimated to be 2.74 eV. Spectroscopic constants of eight low-lying states of SnSe + within 25 000 cm −1 of energy are computed. The electronic states, which include the spin–orbit coupling, have also been investigated. The ground-state spin–orbit splitting is about 1840 cm −1. The radiative lifetimes of B 2Σ +, A 2Σ +, and 2 2Π states of SnSe + are calculated. The effect of the spin–orbit coupling on the A 2Σ +–X 2Π transition has been studied in details. The electric dipole moments of the ion in the low-lying states are reported. The vertical ionization energy of the ground-state SnSe is about 8.93 eV.

Configuration interaction study of single and double dipole plasmon excitations in Na 8

We carry out a microscopic analysis of the ground and excited states of the Na 8 metal cluster within the jellium model. We perform a series of configuration interaction calculations on a Hartree–Fock basis and construct eigenstates of the Hamiltonian which carry up to 4-particle 4-hole components. Based on the analysis of the dipole transition strengths, we single out those states which can be interpreted as the collective dipole plasmon and its double excitations. These modes are found to possess a high degree of harmonicity, deviations from the harmonic limit remaining, however, of the order of 10%.

Does a Sodium Atom Bind to C60?

A Multi-Reference Configuration-Interaction study of the NaC60 system is presented. It is shown that the experimentally measured dipole moment of this system can be explained by the existence of a charge-transfer state of Na(+)C60(-) nature. Moreover, the present work shows that Configuration-Interaction techniques based on local orbitals permit a Multi-Reference treatment of systems containing several tens of atoms.

Configuration interaction study on the conformational and optical properties of excited furan oligomers

Conformational and optical properties of bifuran (BFU) and terfuran (TFU) in their ground states (S 0) and the first singlet excited states (S 1) were investigated by using configuration interaction/singles (CIS), TD-B3LYP and ZINDO approaches. The results show BFU and TFU have both planar and distorted stable conformers in their S 0 states, whereas have planar structure in the S 1 states; BFU and TFU have high inter-ring torsional flexibilities in both the ground-states and the S 1 states; the cis-conformers show different optical properties compared to the trans-conformers. These results should be helpful in further research on oligofurans as molecular electronics.

On the optical properties of thiophene oligomers: configuration interaction study on their ground (S 0) and first singlet excited (S 1) states

In this paper, oligothiophenes (thiophene) n ( n=2–7) were optimized by using Hartree–Fock and restricted configuration interaction/singles (CIS) for their ground states (S 0) and first singlet excited states (S 1) geometries. This was followed by conformational and optical properties investigations using ab initio method in combination with CIS, TD-B3LYP and ZINDO approaches. It is found that bithiophene (BTP) and terthiophene (TTP) are nonplanar in the S 0 states, whereas they almost reach planarity in the S 1 states. The geometry relaxation after excitation contribute much to the Stokes shift observed in their absorption and emission spectra. The global potential energy surface (PES) studies show BTP and TTP have high inter-ring trosional flexibilities, in both the ground-states (S 0) and first excited-states (S 1); the cis-conformers should have evident contribution to the absorption and emission spectra of BTP and TTP molecules. These conclusions are also applicable to larger oligomers by a basic study extended to (thiophene) n ( n=4–7).

Nonlocal model of dissociative electron attachment and vibrational excitation of NO

We present the results of a study of elastic scattering and vibrational excitation of NO by electron impact in the low-energy $(0--2\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ region where the cross sections are dominated by resonance contributions. The $^{3}\ensuremath{\Sigma}^{\ensuremath{-}}$, $^{1}\ensuremath{\Delta}$, and $^{1}\ensuremath{\Sigma}^{+}$ $\mathrm{N}{\mathrm{O}}^{\ensuremath{-}}$ resonance lifetimes are taken from our earlier study [Phys. Rev. A 69, 062711 (2004)], but the resonance energies used here are obtained from different configuration-interaction studies. Here we employ a more elaborate nonlocal treatment of the nuclear dynamics, which is found to remedy the principal deficiencies of the local complex potential model we employed in our earlier study, and gives cross sections in better agreement with the most recent experiments. We also present cross sections for dissociative electron attachment to NO leading to ground-state products, ${\mathrm{O}}^{\ensuremath{-}}(^{2}\mathrm{P})+\mathrm{N}(^{4}\mathrm{S})$. The calculations show that, while the peak cross sections starting from NO in its ground vibrational state are very small $(\ensuremath{\sim}{10}^{\ensuremath{-}20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2})$, the cross sections are extremely sensitive to vibrational excitation of the target and should be readily observable for target NO molecules excited to $\ensuremath{\nu}=10$ and above.

MRCI studies on the electronic spectrum of GeO +

Multireference configuration interaction studies were performed on low-lying states of GeO +. The ground state (GS) was found to be X 2Π, with 1 2Σ + lying 0.22 eV higher. Based on the observed vertical ionization potentials, 1 2Σ + had been assigned as GS in previous experimental work. Calculated R e, T e and D e values, as well as ionization potentials, are in good agreement with experimental data available for X 2Π, 1 2Σ + and 2 2Σ +. Other predicted stable states of GeO + are 1 4Π, 1 4Σ +, at 2.89 and 3.27 eV, respectively, and 1 2Φ, 2 2Δ, 2 2Σ −, 3 2Σ + and 3 2Π, with T e’s in the 5.1–5.4 eV region. The results obtained for GeO + are related to those found for other isovalent diatomic cations.

The crossover between liquid and solid electron phases in quantum dots: A large-scale configuration-interaction study

We study the crossover between liquid and solid electron phases in a two-dimensional harmonic trap as the density is progressively diluted. We infer the formation of geometrically ordered phases from charge distributions and pair correlation functions obtained via a large scale configuration interaction calculation.

Role of the Oxyallyl Substructure in the Near Infrared (NIR) Absorption in Symmetrical Dye Derivatives: A Computational Study

It is well-known from experimental studies that the oxyallyl-substructure-based squarylium and croconium dyes absorb in the NIR region of the spectrum. Recently, another dye has been reported (J. Am. Chem. Soc. 2003, 125, 348) which contains the same basic chromophore, but the absorption is red-shifted by at least 300 nm compared to the former dyes and is observed near 1100 nm. To analyze the reasons behind the large red shift, in this work we have carried out symmetry-adapted cluster-configuration interaction (SAC-CI) studies on some of these NIR dyes which contain the oxyallyl substructure. From this study, contrary to the earlier reports, it is seen that the donor groups do not seem to play a major role in the red-shift of the absorption. On the other hand, on the basis of the results of the high-level calculations carried out here and using qualitative molecular orbital theory, it is observed that the orbital interactions play a key role in the red shift. Finally, design principles for the oxyallyl-substructure-based NIR dyes are suggested.

Improving the efficiency of table-driven CI

Khait and Hoffmann Int J Quantum Chem 2001, 81, 130 have proposed improvements to the efficiency of the table-CI method of Buenker and co-workers for handling spin-adapted functions in configuration-interaction studies of open-shell systems. It is shown that significant further efficiency can be obtained by using methods and formulas published nearly 40 years ago by the present author. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Ground states of BeC and MgC: A comparative multireference Brillouin–Wigner coupled cluster and configuration interaction study

AbstractThe competing X3Σ− and 5Σ− states of the experimentally unknown alkaline–earth metal carbides BeC and MgC are examined with the multireference Brillouin–Wigner coupled cluster method restricted to single and double excitations (MRBW‐CCSD). The results are compared against the traditional single‐reference CCSD approach, as well as with other single and multireference methods. In both molecules, the CCSD 5Σ− ← X3Σ− energy difference is underestimated, leading to an “erroneous” ground‐state prediction in BeC. The MRBW‐CCSD method corrects this anomalous behavior, leading to fair agreement with multireference configuration interaction (MR‐CI) predictions. Our results at the highest levels of theory are extrapolated to the basis set limit, and the core/valence correlation is taken into account, leading to very accurate energetics and spectroscopic constants in both carbides. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Full configuration interaction calculation of singlet excited states of Be3.

The full configuration interaction (FCI) study of the singlets vertical spectrum of the neutral beryllium trimer has been performed using atomic natural orbitals [3s2p1d] basis set. The FCI triangular equilibrium structure of the ground state has been used to calculate the FCI vertical excitation energies up to 4.8 eV. The FCI vertical ionization potential for the same geometry and basis set amounts to 7.6292 eV. The FCI dipole and quadrupole transition moments from the ground state are reported as well. The FCI electric quadrupole moment of the X (3)A(1) (') ground state has been also calculated with the same basis set (Theta(zz)=-2.6461 a.u., Theta(xx)=Theta(yy)=-1/2Theta(zz)). Twelve of the 19 calculated excited singlets are doubly excited states. Most of the states have large multiconfigurational character. These results provide benchmark values for electronic correlation multireference methods. (4ex6MO)CAS-SDCI values for the same energies and properties are also reported.

Ab initio study of AmCl(+): f-f spectroscopy and chemical binding.

A valence full configuration interaction study with a polarized double-zeta quality basis set has been carried out for the lowest 49 electronic states of AmCl(+). The calculations use a pseudopotential treatment for the core electrons and incorporate a one-electron spin-orbit interaction operator. Electrons in the valence s, p, d, and f subshells were included in the active space. The resulting electronic potential energy curves are largely repulsive. The chemical bonding is ionic in character with negligible participation of 5f electrons. The molecular f-f spectroscopy of AmCl(+) arises essentially from an in situ Am(2+) core with states slightly redshifted by the presence of chloride ion. Am(+)+Cl asymptotes which give rise to the few attractive potential energy curves can be predicted by analysis of the f-f spectroscopy of isolated Am(+) and Am(2+). The attractive curves have substantial binding energies, on the order of 75-80 kcal/mol, and are noticeably lower than recent indirect measurements on the isovalent EuCl(+). An independent empirical correlation supports the predicted reduction in AmCl(+) binding energy. The energies of the repulsive curves are strongly dependent on the selection of the underlying atomic orbitals while the energies of the attractive curves do not display this sensitivity. The calculations were carried out using our recently developed parallel spin-orbit configuration interaction software.

Electronic States of TlX (X = As, Sb, Bi): A Configuration Interaction Study

Ab initio based relativistic configuration interaction (CI) calculations are carried out to study the electronic spectrum of TlX (X = As, Sb, Bi) molecules. Potential energy curves and spectroscopic constants of low-lyingelectronic states of these isomers have been computed. Dissociation energies of ground-state molecules are calculated and compared with the experimentally determined values. Effects of the spin-orbit coupling on the spectroscopic properties are studied. The zero-field splitting of the ground state of TIX has been estimated from the spin-orbit CI results. The heavier molecule, TlBi, has a stronger effect of the spin-orbit coupling. Many avoided crossings in the potential curves of Q states of these isomers are reported. Dipole moments of the ground and lowest few bound states, such as 3 Π, 1 Σ + , 1 Δ, and 1 Π, of TIX have been calculated. Transition probabilities of dipole-allowed and spin-forbidden transitions of these molecules are computed. The radiative lifetimes of some of the excited 0 + states are also estimated.

Multireference configuration interaction study of the low-lying electronic states of SiS+

The electronic spectrum of 28Si32S+ has been studied by using relativistic configuration interaction calculations which include effective core potentials. Spectroscopic constants and potential energy curves of low-lying states within 5 eV have been computed. The spin–orbit coupling has also been included in the calculations. The vertical ionization potentials of the ground-state SiS to the ground and a few excited states of SiS+ have been calculated and compared with the experimentally derived values. The transition moment functions of some dipole allowed transitions such as B2Σ+–X2Π, B2Σ+–A2Σ+ etc have been constructed. The dipole moments of three doublets such as X2Π, A2Σ+ and B2Σ+ and three quartets such as 4Π, 4Σ+ and 4Δ of SiS+ are reported. The radiative lifetimes of a couple of excited 2Σ+ states are estimated.