Initio Multireference Configuration Interaction Calculations Research Articles

SCCS−radical: Renner-Teller effect and spin-orbit coupling in the X 2Πu electronic state

SCCS- was detected by laser-induced fluorescence spectroscopy in 2003 (M. Nakajima, Y. Yoneda, Y. Sumiyoshi, T. Nagata, Y. Endo, J. Chem. Phys. 119 (2003) 7805) and its spectrum was analyzed and the results presented together with ab initio calculations data. Symmetrical stretching vibrations were assigned in both the ground X 2?u and the first excited A 2?g electronic states, but no data about spin?orbit splitting and bending vibrational modes were given. In the present work, the vibronic levels in the ground electronic state of SCCS- were calculated by means of a model developed by Peric and coworkers for the treatment of the Renner?Teller effect in any-atomic linear species in its variational form (M. Mitic, R. Rankovic, M. Milovanovic, S. Jerosimic, M. Peric, Chem. Phys. 464 (2016) 55) using the ab initio multireference configuration interaction calculations for obtaining potential energy curves in the Born?Oppenheimer approximation. Additionally, the spin?orbit splitting in the ground state was investigated taking into account the interaction with the first excited state, and the energies obtained from combined treatment of vibronic and spin?orbit interactions in the ground state are reported. Finally, based on the present results, several assignments of unidentified bands reported by Nakajima et al. are proposed.

Full-Dimensional Ab Initio Potential Energy Surface and Vibrational Energy Levels of Li₂H.

We built a full-dimensional analytical potential energy surface of the ground electronic state of Li2H from ca. 20,000 ab initio multi-reference configuration interaction calculations, including core–valence correlation effects. The surface is flexible enough to accurately describe the three dissociation channels: Li (2s 2S) + LiH (1Σ+), Li2 (1Σg+) + H (1s 2S) and 2Li (2s 2S) + H (1s 2S). Using a local fit of this surface, we calculated pure (J = 0) vibrational states of Li2H up to the barrier to linearity (ca. 3400 cm−1 above the global minimum) using a vibrational self-consistent field/virtual state configuration interaction method. We found 18 vibrational states below this barrier, with a maximum of 6 quanta in the bending mode, which indicates that Li2H could be spectroscopically observable. Moreover, we show that some of these vibrational states are highly correlated already ca. 1000 cm−1 below the height of the barrier. We hope these calculations can help the assignment of experimental spectra. In addition, the first low-lying excited states of each B1, B2 and A2 symmetry of Li2H were characterized.

An investigation of the sites occupied by atomic barium in solid xenon-A 2D-EE luminescence spectroscopy and molecular dynamics study.

A detailed characterisation of the luminescence recorded for the 6p 1P1-6s 1S0 transition of atomic barium isolated in annealed solid xenon has been undertaken using two-dimensional excitation-emission (2D-EE) spectroscopy. In the excitation spectra extracted from the 2D-EE scans, two dominant thermally stable sites were identified, consisting of a classic, three-fold split Jahn-Teller band, labeled the blue site, and an unusual asymmetric 2 + 1 split band, the violet site. A much weaker band has also been identified, whose emission is strongly overlapped by the violet site. The temperature dependence of the luminescence for these sites was monitored revealing that the blue site has a non-radiative channel competing effectively with the fluorescence even at 9.8 K. By contrast, the fluorescence decay time of the violet site was recorded to be 4.3 ns and independent of temperature up to 24 K. The nature of the dominant thermally stable trapping sites was investigated theoretically with Diatomics-in-Molecule (DIM) molecular dynamics simulations. The DIM model was parameterized with ab initio multi-reference configuration interaction calculations for the lowest energy excited states of the Ba⋅Xe pair. The simulated absorption spectra are compared with the experimental results obtained from site-resolved excitation spectroscopy. The simulations allow us to assign the experimental blue feature spectrum to a tetra-vacancy trapping site in the bulk xenon fcc crystal-a site often observed when trapping other metal atoms in rare gas matrices. By contrast, the violet site is assigned to a specific 5-atom vacancy trapping site located at a grain boundary.

Femtosecond real-time probing of reactions MMXVII: The predissociation of sodium iodide in the A 0+ state

We revisit the femtosecond transition-state spectroscopy of sodium iodide taking advantage of modern lasers and pulse-shaping to better map the low-lying electronic states, some forming predissociative wells through curve crossings. We also carry out high-level ab initio multi-reference configuration interaction calculations including spin-orbit coupling terms and using large correlation-consistent basis sets to arrive at accurate ground- and excited-state potential energy curves of NaI. Density matrix calculations employing vibrational wave functions determined from the ab initio X 0+ and A 0+ potentials are used to simulate time dependent wave packet dynamics of NaI pumped to the A 0+ state.

Magnetic Excitations and Electronic Interactions in Sr_{2}CuTeO_{6}: A Spin-1/2 Square Lattice Heisenberg Antiferromagnet.

Sr_{2}CuTeO_{6} presents an opportunity for exploring low-dimensional magnetism on a square lattice of S=1/2 Cu^{2+} ions. We employ abinitio multireference configuration interaction calculations to unravel the Cu^{2+} electronic structure and to evaluate exchange interactions in Sr_{2}CuTeO_{6}. The latter results are validated by inelastic neutron scattering using linear spin-wave theory and series-expansion corrections for quantum effects to extract true coupling parameters. Using this methodology, which is quite general, we demonstrate that Sr_{2}CuTeO_{6} is an almost ideal realization of a nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling of 7.18(5)meV.

Study of the electronic and rovibronic structure of the X ²Σ⁺, A ²Π, and B ²Σ⁺ states of AlO.

The electronic structure of the X (2)Σ(+), A (2)Π, and B (2)Σ(+) states of aluminum monoxide (AlO) are studied via ab initio multi-reference configuration interaction calculations. Core correlation corrections, several basis sets, and active space choices are considered. Angular momentum and spin-orbit coupling terms are obtained at different levels of theory. The resulting ab initio curves are used to solve the associated rovibronic problem for the total angular momentum J up to 112.5 and then also refined by fitting to the experimental wavenumbers available in the literature, reproducing them with the root-mean-square error of 0.07 cm(-1). Theoretical rovibronic energy levels of AlO in its X (2)Σ(+), A (2)Π, and B (2)Σ(+) electronic states are presented including those from the X - B blue-green system.

Potential energy surfaces and quasiclassical trajectory study of the O + H2(+)→ OH(+) + H, OH + H(+) proton and hydrogen atom transfer reactions and isotopic variants (D2(+), HD(+)).

The rate constants (k; T: 200-900 K) and cross-sections (σ; Ecol: 0.010-0.50 eV) of the O + H2(+)→ OH(+) + H (1), OH + H(+) (2) reactions, which occur on the ground (1(2)A'') and first excited (1(2)A') potential energy surfaces (PESs), respectively, were investigated for the first time, considering also the rate constants for D2(+) and HD(+). Ab initio multireference configuration interaction calculations were performed on both barrierless PESs (where the minimum energy path involves the insertion of the O atom into the middle of the H2(+) bond), and suitable analytical expressions were developed for the first time and used in quasiclassical trajectory (QCT) calculations. k(1) ≈ 3k(2) independent of the isotopic variant, k(H2(+)) > k(HD(+)) > k(D2(+)) for , and the intermolecular and intramolecular isotopic effects are essentially independent of T. Comparison with the Langevin-Gioumousis-Stevenson (LGS) simple capture model shows that these results are similar to the QCT ones, especially for ; and the isotopic effects are coincident with the QCT ones for both reactions. For O + H2(+), σ(1) ≈ 3σ(2) at Ecol≤ 0.10 eV, and σ(1) = 1.5σ(2) at 0.40 and 0.50 eV. The larger value of σ(1(2)A'') with respect to σ(1(2)A') arises from the larger value of bmax(1(2)A'') with respect to bmax(1(2)A'), and this results from the more attractive character of the former PES. Besides, the reaction probabilities are quite large [0.78-0.98 (1(2)A'') and 0.78-0.93 (1(2)A')], and the decreasing trend of both cross-sections as Ecol increases arises from the barrierless character of both PESs. We expect that these results (in particular, the competition between proton transfer and hydrogen atom transfer) will encourage experimentalists to carry out investigations on this interesting reaction.

Coupled potential energy surface for the F(2P) + CH4 → HF + CH3 entrance channel and quantum dynamics of the CH4 · F− photodetachment

An approach to construct vibronically and spin-orbit coupled diabatic potential energy surfaces (PESs) which describe all three relevant electronic states in the entrance channels of the X(P) + CH4 →HX + CH3 reactions (with X=F((2)P), Cl((2)P), or O((3)P)) is introduced. The diabatization relies on the permutational symmetry present in the methane molecule and results in diabatic states which transform as the three p orbitals of the X atom. Spin-orbit coupling is easily and accurately included using the atomic spin-orbit coupling matrix of the isolated X atom. The method is applied to the F + CH4 system obtaining an accurate PES for the entrance channel based on ab initio multi-reference configuration interaction (MRCI) calculations. Comparing the resulting PESs with spin-orbit MRCI calculations, excellent agreement is found for the excited electronic states at all relevant geometries. The photodetachment spectrum of CH4·F(-) is investigated via full-dimensional (12D) quantum dynamics calculations on the coupled PESs using the multi-layer multi-configurational time-dependent Hartree approach. Extending previous work [J. Palma and U. Manthe, J. Chem. Phys. 137, 044306 (2012)], which was restricted to the dynamics on a single adiabatic PES, the contributions of the electronically excited states to the photodetachment spectrum are calculated and compared to experiment. Considering different experimental setups, good agreement between experiment and theory is found. Addressing questions raised in the previous work, the present dynamical calculations show that the main contribution to the second peak in the photodetachment spectrum results from electron detachment into the electronically excited states of the CH4F complex.

Configuration interaction study including the effects of spin–orbit coupling for the electronic states of the LiX molecules (X = C, Si, Ge, Sn)

Ab initio multireference configuration interaction calculations including spin–orbit coupling effects have been carried out for four LiX molecules (X = C, Si, Ge and Sn). Potential energy curves of the ground and low-lying excited states have been obtained in each case as well as the corresponding spectroscopic constants. Transition moments have also been computed in order to give estimates of the radiative lifetimes of the excited states for each system. Trends in a variety of quantities such as T e values, spin–orbit splittings, equilibrium bond lengths and vibrational frequencies for this series of molecules are discussed in detail and comparison with the corresponding data reported earlier for the PbLi system is also made.

The valence shell electronic states of trimethylene oxide studied by photoabsorption and ab initio multireference configuration interaction calculations

The absolute photoabsorption cross section of trimethylene oxide has been measured between threshold and 30 eV using monochromated synchrotron radiation. Below the ionization threshold the spectrum exhibits numerous sharp peaks associated with Rydberg states belonging to series converging onto the limit. At excitation energies above the ionization threshold at 9.680 eV intravalence transitions play a dominant role, resulting in the appearance of prominent broad bands. Ab initio multireference configuration interaction calculations have been performed to obtain excitation energies for valence electron transitions into Rydberg or virtual valence orbitals. These theoretical predictions have enabled assignments to be proposed for most of the structure due to Rydberg series converging onto the limit. The calculations show that configuration interaction is important in the description of some of the excited states. The photoabsorption spectrum of trimethylene oxide has been compared with those of ethylene oxide, ethylene sulphide and trimethylene sulphide to identify and characterize the principal Rydberg excitations that these related three- or four-membered ring molecules have in common.

The valence shell electronic states of trimethylene sulphide studied by photoabsorption and ab initio multireference configuration interaction calculations

The absolute photoabsorption cross section of trimethylene sulphide has been measured between threshold and 30 eV using monochromated synchrotron radiation. Below the ionization threshold the spectrum exhibits numerous sharp peaks associated with Rydberg states belonging to series converging onto the limit. At excitation energies above the ionization threshold at 8.655 eV intravalence transitions play a dominant role, resulting in the appearance of prominent broad bands. Ab initio multireference configuration interaction calculations have been performed to obtain excitation energies for valence electron transitions into Rydberg or virtual valence orbitals. These theoretical predictions have enabled assignments to be proposed for most of the structure due to Rydberg series converging onto the limit. The calculations show that configuration interaction is important in the description of many of the excited states. This is particularly evident close to threshold where the theoretical results suggest that the observed, highly irregular, structure may be attributed to two Rydberg transitions and an intravalence transition. The calculations also establish that both of the dipole allowed 4b1 → nda1 B1 Rydberg series should contribute to the absorption spectrum.

The electronic states of ethylene oxide studied by photoabsorption and ab initio multireference configuration interaction calculations

The absolute photoabsorption spectrum of ethylene oxide (C2H4O) has been measured between onset and 30 eV, using monochromated synchrotron radiation. Below the ionization threshold (10.56 eV) the spectrum is dominated by sharp peaks related to excitation of Rydberg series converging on the first ionization energy. Above the ionization threshold, valence-excited states, which give rise to broad bands, predominate. Underlying Rydberg states are signalled in weak vibrational structure on the valence bands. Ab initio multireference configuration interaction (MRDCI) calculations have been carried out to assist in assignment of the excited states, both valence and Rydberg. The lowest-lying valence state is electric-dipole forbidden; the first optically allowed valence state lies close to the ionization onset. The spectrum of the oxide is compared with those of cyclopropane (C3H6) and ethylene sulphide (C2H4S).

Theoretical study on the structure of the ground state and photo-induced metastable states of [M(CN)5NO]2− (M = Ru, Fe), and mechanism of the photo-rearrangement among them

Abstract In this note, we reviewed theoretical investigations of the photo-induced metastable states and discussed possible photo-rearrangement paths in [M(CN)5NO]2− (M = Fe and Ru). Using calculated results of ab initio multi-reference single and double excitation configuration interaction calculations with Davidson type quadruple correction (MRSDCI + Q) and those of density fuctional theory (DFT), we showed that the two metastable states (MS1 and MS2) were local minima of the electronic ground state potential surface. The theoretical calculations supported the experimental observation of the structure of the three isomers (GS, MS1 and MS2), relative stability of MS1 and MS2. We discussed the character of the lower electronic excited states and excitation energies of the three isomers of both compounds. Based on these results and excited state adiabatic potential energy surfaces, possible mechanism of photo-rearrangement among GS, MS1, and MS2 were discussed with the aid of our results on [Mn(CN)5NO]3− of which no photo-rearrangement has been reported.

Ab initio molecular orbital study of ground and low-lying electronic states of NiCN

The electronic ground state and some low-lying excited states of NiCN have been studied by ab initio multi-reference single and double excitation configuration interaction calculations (MR-SDCI) with Davidson’s correction Q and Cowan–Griffin’s relativistic corrections E rel. The electronic ground state of NiCN is 2Δ i and the equilibrium geometry is linear with bond lengths of r e(Ni–C) = 1.8141 Å and r e(C–N) = 1.1665 Å. According to multi-reference averaged quadratic coupled-cluster theory (MR-AQCC) with E rel, the first excited state is 2Σ +, separated from the ground state by 446 cm −1, the second and third excited states are 2Π i (at 1594 cm −1) and 4Δ i (at 8930 cm −1), respectively, and other quartet states follow. The relative stabilities and the molecular properties of the low- and high-spin states having the same electronic total angular momentum are shown to be determined by dynamical electron correlation effects. A spin–orbit interaction scheme for the lowest doublet manifold is proposed. The predicted term values (in cm −1) are 0 ( X ∼ 2 Δ 5 / 2 ) , 793 ( 2 Σ 1 / 2 + ) , 891 ( 2Δ 3/2), 2321 ( 2Π 3/2), and 2773 ( 2Π 1/2). Thus, an unassigned state reported by Kingston et al. [C.T. Kingston, A.J. Merer, T.D. Varberg, J. Mol. Spectrosc. 215 (2002) 106] at 755 cm −1 could be the 1 2 Σ 1 / 2 + state.

The valence shell electronic states of ethylene sulphide studied by photoabsorption and ab initio multireference configuration interaction calculations

The absolute photoabsorption cross section of ethylene sulphide has been measured between threshold and 30 eV using monochromated synchrotron radiation. Below the ionisation threshold the spectrum exhibits numerous sharp peaks associated with Rydberg states belonging to series converging onto the X ∼ 2 B 1 limit. At excitation energies above the ionisation threshold at 9 eV intravalence transitions play a dominant role, resulting in the appearance of prominent broad bands. Much weaker structure ascribed to Rydberg states associated with excitation from the 4b 2, 8a 1 or 2b 1 orbitals is also discernible. Ab initio multireference configuration interaction calculations have been performed to obtain excitation energies for valence electron transitions into Rydberg or virtual valence orbitals. These theoretical predictions have enabled assignments to be proposed for most of the structure due to Rydberg series converging onto the X ∼ 2 B 1 limit. The calculations show that configuration interaction is important in the description of many of the excited states.

Photodissociation of small carbonaceous molecules of astrophysical interest

Astronomical observations have shown that small carbonaceous molecules can persist in interstellar clouds exposed to intense ultraviolet radiation. Current astrochemical models lack quantitative information on photodissociation rates in order to interpret these data. We here present ab initio multi-reference configuration–interaction calculations of the vertical excitation energies, transition dipole moments and oscillator strengths for a number of astrophysically relevant molecules: C 3, C 4, C 2H, l- and c-C 3H, l- and c-C 3H 2, HC 3H, l-C 4H and l-C 5H. Highly excited states up to the 9th root of each symmetry are computed, and several new states with large oscillator strengths are found below the ionization potentials. These data are used to calculate upper limits on photodissociation rates in the unattenuated interstellar radiation field by assuming that all absorptions above the dissociation limit lead to dissociation.

The electronic states of cyclopropane studied by VUV absorption and ab initio multireference configuration interaction calculations

The absolute photoabsorption cross-section of cyclopropane has been measured between threshold and 30 eV using monochromated synchrotron radiation. The spectrum is dominated by prominent broad peaks arising from electronic transitions into valence-excited states. Much weaker structure ascribed to Rydberg states associated with excitation from the 3e′ or 1a″2 orbitals is also discernible. Some of these Rydberg states display vibrational progressions which can be correlated with similar structure observed in the corresponding photoelectron spectrum. Ab initio multireference configuration interaction calculations have been performed to obtain excitation energies for valence electron transitions into Rydberg or virtual valence orbitals. These theoretical predictions have enabled assignments to be proposed for most of the observed absorption bands.

Theoretical study of nonadiabatic interactions, radiative lifetimes and predissociation lifetimes of excited states of BH

Ab initio multireference configuration interaction calculations have been carried out on the lower-lying electronic states of BH and their nonadiabatic interactions. The ab initio data have been included in subsequent calculations involving solution of the complex eigenvalue Schrödinger equation to determine predissociation widths and lifetimes of vibrational-rotational levels of these states. Secondly, previously calculated ab initio data on the Rydberg states and their nonadiabatic interactions have been included in multi-state vibrational calculations on the 3p and 3d complexes in BH and BD. The results are in good agreement with the experimental analysis of the 3p and 3d spectra in BH and BD. Furthermore, interaction of the 3d states with the neighbouring 4s state is also found to be important.

Potential energy surfaces and vibrational energy levels of DCCl and HCCl in three low-lying states

We present ab initio multireference configuration interaction (MRCI) calculations for the potential energy surfaces of HCCl in its three low-lying electronic states ( and ), and for the spin–orbit coupling between the and states. The two singlet states become a degenerate 1Δ state in collinear geometries. The potential energy surfaces are interpolated from 6075 MRCI energy points. The final surfaces are slightly adjusted using a coordinate and energy scaling approach. The resulting T e value is 2122.0 cm−1 for and 12209.8 cm−1 for . Vibrational energy levels of DCCl and HCCl are computed for the three states taking into account the Renner–Teller effect and the spin–orbit coupling. The calculated vibronic energy levels are in good agreement with the available experimental values. It is found that the spin–orbit effect is pronounced in the and states.

Electron-impact dissociation of D13CO+ molecular ions to 13CO+ ions

Absolute cross sections for electron-impact dissociation of D13CO+ ions have been measured over a collision energy range from 4 to 100 eV with a crossed electron–ion beams method. Total experimental uncertainties are about 16% near the cross section peak. The role of resonant and direct dissociative excitation for energies below 21 eV is discussed in light of the energy levels and photo-dissociation cross sections of the HCO+ formyl cation predicted by ab initio multi-reference configuration interaction calculations.