Multireference Configuration Interaction Study Research Articles

A multireference configuration interaction study with singles and doubles of some mesoionic rings: reaction and activation free energies for the ring‐opening reaction

AbstractFor the first time, multireference configuration interaction with singles and doubles (MR‐CISD) calculations (including extensivity corrections, MR‐CISD+Q) have been performed to study the ring‐opening reactions of the following mesoionic rings and their 2, 3, and 4 methyl‐substituted derivatives: (a) 1,3‐oxazol‐5‐one, (c) 1,3‐oxazol‐5‐thiolate, and (d) 1,3‐thiazol‐5‐thiolate. The ring‐opening reaction of the parent 1,3‐thiazol‐5‐one mesoionic ring (b) has also been studied. The effect of methyl and S replacement on the reaction and activation free energies (ΔG and ΔG†, respectively) is studied. For a, the effect of methyl replacement on C2 is almost negligible, while on N3 and C4, it is significantly larger, especially in the latter position. The open structure (ketene form) of a is considerably more stable, and the replacement of the exocyclic O by S stabilizes the ketene tautomer even more and increases ΔG† considerably. On the other hand, replacement of the endocyclic O by S (yielding b) completely prevents the formation of the open structure, while replacement of both endo and exo atoms strongly stabilizes the cyclic structure and leads to a remarkably high value of ΔG†. In some cases, the relative polarity of the stationary points is used to estimate how ΔG and ΔG† are expected to change as one goes from the gas phase to aprotic polar solvents. A linear correlation between the multiconfigurational character of the mesoionic rings and ΔG† has been observed, and such correlation is used to estimate the ΔG† values for the methyl‐substituted c and d. The largest barrier (13.78 kcal/mol) has been obtained for the 4‐methyl‐1,3‐thiazol‐5‐thiolate (4‐m‐d).

Multireference configuration interaction study on the ground and excited electronic states of the AlO+ molecule

High-level ab initio calculations on the electronic states of AlO+ cation have been performed with the explicitly correlated multi-reference configuration interaction (MRCI-F12) method. The potential energy curves (PECs) of 24 electronic states have been obtained, most of which are reported for the first time. From the computed PECs, the precise spectroscopic constants of the bound states are determined. Our calculations confirm that the ground state of AlO+ cation is the X1Σ+ state. The permanent dipole moments (PDMs) functions of the selected bound states are computed. The spin–orbit (SO) matrix elements between the electronic states involved in the crossing region of the PECs are calculated to analyze the predissociation mechanisms of the X1Σ+, A1Π, and 21Σ+ states. Finally, the transition properties of four spin-allowed transitions are predicted, including the transition dipole moments (TDMs), Franck-Condon Factors (FCFs), and the radiative lifetimes. This work should enhance our understanding on the electronic structure and spectroscopy of AlO+ cation.

Multireference configuration interaction study of the 21 low-lying states of the OF radical

This paper calculated the potential energy curves of 21Λ-S and 42Ω states of the OF radical. The 21Λ-S states were the X2Π, A2Σ−, B2Σ−, C2Δ, D2Σ+, E2Σ+, 22Π, a4Σ−, b4Δ, 24Σ−, 14Σ+, 14Π, 24Π, 32Σ+, 32Σ−, 32Π, 42Π, 52Π, 22Δ, 32Δ, and 12Φ, which arose from the first two dissociation limits. The 42Ω states were generated from these Λ-S states. All the potential energy curves were calculated with the CASSCF method, which was followed by the icMRCI+Q approach. The 14Π, 24Π, 22Π, 42Π, 52Π, 32Σ+, and 32Δ states were repulsive whether the spin-orbit coupling effect included or not, but the A2Σ−, D2Σ+, 32Σ−, 22Δ, and 12Φ states became repulsive with the spin-orbit coupling effect included. Only the 16Ω states were bound. With the spin-orbit coupling effect accounted for, the X2Π state was inverted among the bound states; the X2Π, a4Σ−, and E2Σ+ states were strongly bound; and the 32Π, b4Δ, B2Σ−, C2Δ, 24Σ−, and 14Σ+ states were very weakly bound. The spectroscopic and vibrational properties were determined. Franck–Condon factors of some transitions were evaluated. The spin-orbit coupling effect on the spectroscopic parameters and vibrational properties was discussed. It is very difficult to explore the X2Π, a4Σ−, and E2Σ+ states by observing the electronic transitions between them because all these strong transitions originating only from the highly-vibrational states of the X2Π or a4Σ− state. It is also very hard to detect the 32Π, b4Δ, B2Σ−, C2Δ, 24Σ−, and 14Σ+ states by observing the transitions originating from these states, because these states are very weakly bound and unstable, though some transitions originating from them are very strong. These results can well explain why the OF radical is very difficult to detect in a spectroscopic experiment by observing the electronic transitions between different states.

Multireference configuration interaction study of ground and low-lying excited states of BrO radical

This work explored the spectroscopic parameters and vibrational properties of 12 states of BrO radical. The 12 states were X2Π, A2Π, 12Δ, 12Σ+, 12Σ−, 22Σ−, a4Σ−, 24Σ−, 14Σ+, 14Δ, 14Π, and 24Π, which were generated from the first dissociation limit, Br(2Pu)+O(3Pg). The potential energy curves were calculated with the CASSCF method, which was followed by the icMRCI approach with Davidson correction. Of the 12 electronic states, the 24Π state was repulsive. Each state had the single well and only the A2Π state had one barrier. The 12Δ, 12Σ+, 12Σ−, 22Σ−, 24Σ−, 14Σ+, 14Δ and 14Π states had only several vibrational levels and were very weakly-bound states, which well depths were less than 110cm−1. To improve the quality of potential energy curves, core-valence correlation and scalar relativistic corrections as well as the extrapolation to the complete basis set limit were included. The spectroscopic parameters and vibrational properties were determined. The transition dipole moments between two electronic states and Franck-Condon factors of some transitions were evaluated. The spin-orbit coupling effect on the spectroscopic parameters of the X2Π and A2Π states was discussed. The spectroscopic parameters and vibrational properties reported in this paper can be expected to be reliably predicted ones.

Multireference configuration interaction study of ground and low-lying excited states of the AlC radical

ABSTRACTThis work explored the spectroscopic parameters and vibrational properties of the 21 Λ–S and 42 Ω states of the AlC radical. The PECs were calculated with the CASSCF method, which was followed by the icMRCI+Q approach. The A4Π, a2Π, 52Π, 22Δ, and 12Φ states as well as the first well of B4Σ− state were inverted with the spin–orbit coupling (SOC) effect included; the 14Δ, 14Σ+, and 22Σ− states as well as the second wells of the B4Σ−, 22Σ+, 32Σ+, 42Π and 52Π states were weakly bound, which well depths were less than 650 cm−1; the B4Σ−, 22Σ+, 32Σ+, 42Π, 52Π, and 22Δ states had double wells and the second wells of these states except for B4Σ− had only several vibrational states; the avoided crossings existed between the 22Σ+ and 32Σ+ states, the 32Σ+ and 42Σ+ states, the B4Σ− and 34Σ− states, the 22Δ and 32Δ states, the 42Π and 52Π states, the 52Π and 62Π states, as well as the 24Π and 34Π states. The extrapolation scheme, core–valence correlation and scalar relativistic corrections were included. The spectroscopic parameters and vibrational properties were determined. The TDM curves between two different Λ–S states were calculated and Franck–Condon factors of some transitions were evaluated. The SOC effect on the spectroscopic and vibrational properties was evaluated.

Multireference configuration interaction study of the 27 low-lying states of the PF+ cation

This paper studied the spectroscopic parameters and vibrational properties of 27 Λ-S and 60Ω states of PF+ cation. The 27 Λ-S states were the X2Π, A2Σ+, B2Π, C2Σ−, D2Δ, a4Σ−, b4Π, c4Σ+, d4Δ, 22Σ+, 32Σ+, 42Σ+, 22Σ−, 32Σ−, 32Π, 42Π, 52Π, 62Π, 22Δ, 32Δ, 12Φ, 24Σ−, 34Σ−, 24Π, 34Π, 16Σ−, and 16Π, which were generated from the first four dissociation limits. The 60Ω states were produced from the 27 Λ-S states. All the potential energy curves were calculated with the CASSCF method, which was followed by the icMRCI+Q approach. The a4Σ−, b4Π, and D2Δ states were inverted with the spin-orbit coupling effect accounted for. The 24Π, 24Σ−, 22Δ, 32Δ, 32Σ+, 42Σ+, 12Φ, and 22Σ− states were repulsive whether the spin-orbit coupling effect was included or not, but the 52Π and D2Δ states became repulsive only with the spin-orbit coupling effect included. The C2Σ− state was very weakly bound. The a4Σ− state had one barrier. The avoided crossings existed between the a4Σ− and 24Σ− states, the 22Σ+ and 32Σ+ states as well as the D2Δ and 22Δ states. Core-valence correlation and scalar relativistic corrections were taken into account. The extrapolation to the complete basis set limit was done. The spectroscopic parameters and vibrational properties were determined. The transition dipole moments were calculated and the Franck-Condon factors of some electric dipole transitions were evaluated. The spin-orbit coupling effect on the spectroscopic and vibrational properties was discussed.

Ab initio coupled-cluster and multi-reference configuration interaction studies of the low-lying electronic states of 1,2,3,4-cyclobutanetetraone

ABSTRACTThe four, closely spaced, lowest energy electronic states of the challenging, D4h-symmetric, 1,2,3,4-cyclobutanetetraone (C4O4) molecule have been investigated using high-level ab initio methods. The calculated states include the closed-shell singlet 8π(1A1g) state, the singlet 10π(1A1g) state, in which the π-type lowest unoccupied molecular orbital (LUMO) of the 8π(1A1g) reference is doubly occupied and the σ-type highest occupied molecular orbital (HOMO) is empty, and the open-shell singlet and triplet states, designated as 9π(1B2u) and 9π(3B2u), respectively, originating from single occupancy of the HOMO and LUMO. Our focus is on single-reference coupled-cluster (CC) approaches capable of handling electronic near-degeneracies in diradicals, especially the completely renormalised CR-CC(2,3) and active-space CCSDt methods, along with their CCSD and EOMCCSD counterparts. The internally contracted multi-reference configuration interaction calculations with a quasi-degenerate Davidson correction are performed as well. Our computations demonstrate that the state ordering is 9π(3B2u) < 8π(1A1g) < 9π(1B2u) < 10π(1A1g) and that the 8π(1A1g) − 9π(3B2u) gap is in the 7–11 kJ/mol range, in reasonable agreement with the negative ion photoelectron spectroscopy measurements, which give 6.27 ± 0.5 kJ/mol. In addition to the theory level used, geometry relaxation and basis set play a significant role in determining the state ordering and energy spacings. In particular, it is unsafe to use lower level, non-CC geometries and smaller basis sets.

On the photophysics of four heteroleptic iridium(III) phenylpyridyl complexes investigated by relativistic multi-configuration methods

ABSTRACTIn this work, we present a detailed analysis of the photophysical properties of four phosphorescent iridium(III) complexes, i.e. trans-N,N- and cis-N,N-(ppy)2IrIII(acac) as well as their fluorinated derivatives trans-N,N- and cis-N,N-(F2ppy)2IrIII(acac). These properties include absorption and emission characteristics, intersystem crossing rates from the lowest singlet excited state, phosphorescence lifetimes of the individual triplet sublevels as well as the orientations of the transition dipole vectors. To this end, we have carried out combined density functional theory and multi-reference configuration interaction studies including spin–orbit coupling by perturbational as well as variational procedures. For the experimentally known complexes, we observe excellent agreement between our computed data and literature data. Also the blueshifts of the emission maxima occurring upon fluorination of the (ppy)2Ir(acac) compounds are well reproduced. To our surprise, we find the experimentally not yet investigated cis-N,N-(F2ppy)2Ir(acac) isomer to be thermodynamically more stable than the well-known blue phosphorescent emitter trans-N,N-(F2ppy)2Ir(acac).

Multireference configuration interaction study of dichlorocarbene

Dichlorocarbene is an important reactive chemical intermediate, which has received much investigation. However, the energy, structure, and dynamics of the excited states are not well understood. Here, we investigated the equilibrium geometries and energies of seven electronic states, X̃1A1, Ã1B1, ã3B1, B̃1A2, b̃3A2, C̃1B2 and c̃3B2 of CCl2 using internally contracted multireference configuration interaction method including Davidson correction (icMRCI+Q) with different basis sets aug-cc-pV(X+d)Z(X=T,Q,5), and the effects of different basis sets on the geometries and energies. For the first time, the theoretical studies of high excited singlet and triplet states of CCl2 that are related to the lowest three dissociation limits were carried out at the icMRCI+Q/aug-cc-pV(5+d)Z level. The results were compared with the previous experimental and theoretical data where available. Our results will add some understanding and shed some light on the structures and dynamics of electronic states of CCl2 radical.

The electronic structure of VO in its ground and electronically excited states: A combined matrix isolation and quantum chemical (MRCI) study.

The electronic ground and excited states of the vanadium monoxide (VO) molecule were studied in detail. Electronic absorption spectra for the molecule isolated in Ne matrices complement the previous gas-phase spectra. A thorough quantum chemical (multi-reference configuration interaction) study essentially confirms the assignment and characterization of the electronic excitations observed for VO in the gas-phase and in Ne matrices and allows the clarification of open issues. It provides a complete overview over the electronically excited states up to about 3 eV of this archetypical compound.

Ground and excited states of [Be–Xe]+: A multireference configuration interaction study

Potential energy curves (PECs) for the first three dissociation channels of [Be–Xe]+ have been calculated at the MRCI level of theory with Davidson and relativistic correction. Spin–orbit coupling is considered by the state interaction approach using the full Breit-Pauli Hamiltonian to analyze the interactions between Σ and Π electronic states in PECs crossing regions. Curves avoided crossings must be considered for A2Π and 22Π, 22Σ+ and 32Σ+ states to accurately estimate of spectroscopic parameters. Based on the PECs, spectroscopic parameters are determined. Finally, the transition dipole moments and radiative lifetimes of all possible transitions are predicted for future laboratory researches.

Profiling the binding motif between Be and Mg in the ground state via a single-reference coupled cluster method

We present a study on the performance of our iterative triples correction for the coupled cluster singles and doubles excitations (CCSDT-1a+d) method for computation of potential energy surface (PES), spectroscopic constants, and vibrational spectrum for the ground state (X1Σ+) BeMg, where the ostensible inadequacy of the CCSD and CCSD(T) methods is quite expected. We compare our results with those obtained using state-of-the-art multireference configuration interaction (MRCI) investigations reported earlier by Kerkines and Nicolaides. Our estimated dissociation energy (417.37 cm−1), equilibrium distance (3.285 Å), and vibrational frequency (82.32 cm−1) are in good agreement with recent results of advanced MRCI calculations for X1Σ+ BeMg PES, which exhibits a shallow well of 469.4 cm−1 with a minimum at 3.241 Å and a harmonic vibrational frequency of 85.7 cm−1. Very weakly bound nature of X1Σ+ BeMg is clearly reflected from these values. In accord with MRCI studies, a comparison of BeMg with iso-valence weakly bound ground-state species, Be2 and Mg2, suggests that its characteristics do not exhibit any resemblance to Be2 rather, it shows a close kinship to Mg2. The agreement of our derived vibrational levels with those obtained via the high-level MRCI calculations is very encouraging reflecting the potential of the suitably modified single-reference coupled cluster (SRCC) method, CCSDT-1a+d as a tool for the study of multireference van der Waals systems.

Multireference configuration interaction study on the potential energy curves and radiative lifetimes of low-lying excited states of CdH+ cation

Ab initio calculations on CdH+ cation have been performed with the high-level relativistic MRCI+Q method. The potential energy curves of seven low-lying Λ–S states, correlated with the dissociation limits Cd+(2Sg)+H(2Sg), Cd(1Sg)+H+(1Sg), and Cd+(2Pu)+H(2Sg), are computed. And the accurate spectroscopic constants of the bound Λ–S are determined. The permanent dipole moments (PDMs) of Λ–S states as well as the spin–orbit matrix elements are evaluated. The results indicate that the abrupt changes of PDMs and the spin–orbit matrix elements are attributed to the change of the electronic configurations at the avoided crossing point. After considering the spin–orbit coupling (SOC) effect, the 12Ω states generated from the Λ–S states are investigated. It is indicated that the SOC effect is substantial for CdH+, leading to the double-well potential of (3)0+ state. Finally, the transition properties of several transitions are reported, including the transition dipole moments, Franck–Condon factors, and radiative lifetimes.

A multireference configuration interaction study of the photodynamics of nitroethylene.

Extended multireference configuration interaction with singles and doubles (MR-CISD) calculations of nitroethylene (H2C=CHNO2) were carried out to investigate the photodynamical deactivation paths to the ground state. The ground (S0) and the first five valence excited electronic states (S1–S5) were investigated. In the first step, vertical excitations and potential energy curves for CH2 and NO2 torsions and CH2 out-of-plane bending starting from the ground state geometry were computed. Afterward, five conical intersections, one between each pair of adjacent states, were located. The vertical calculations mostly confirm the previous assignment of experimental spectrum and theoretical results using lower-level calculations. The conical intersections have as main features the torsion of the CH2 moiety, different distortions of the NO2 group and CC, CN, and NO bond stretchings. In these conical intersections, the NO2 group plays an important role, also seen in excited state investigations of other nitro molecules. Based on the conical intersections found, a photochemical nonradiative deactivation process after a π–π* excitation to the bright S5 state is proposed. In particular, the possibility of NO2 release in the ground state, an important property in nitro explosives, was found to be possible.

Combined Experimental and Ab Initio Multireference Configuration Interaction Study of the Resonant Inelastic X-ray Scattering Spectrum of CO2

The fundamental problem of the symmetry breaking in the resonant inelastic X-ray scattering (RIXS) of the CO2 gas molecule is studied. The measurements were performed under catalytically relevant c ...

MRCI study on potential energy curves and spectroscopic properties of HF molecule

The potential energy curves of the low-lying electronic states of Hydrogen fluoride (HF) have been investigated by using multireference configuration interaction (MRCI) approach in combination with cc-pV5Z basis sets for H and aug-cc-pV6Z basis sets for F. Three singlet states (X1Σ+,B1Σ+, 11Δ), two triplet valence states (3ΠV,3ΣV+) and five triplet Rydberg states (3ΣR1+,3ΠR1,3ΔR1,3ΣR1-,3ΠR2) of HF molecular are identified. 3ΠV and 3ΣV+ states are repulsive, the spectroscopic parameters (De, Re, ωe, Be and Te) of the bound states are derived in the present work. We find that the order of energy levels of 3ΣR1+,3ΠR1,3ΔR1 electronic states is different with other literatures, 3ΣR1+ is the lowest in these three states, which undergoes a strongly avoided crossing with the valence state 3ΣV+. The 3ΣR1- state is studied at the first time. In addition, the dipole moments (DM), transition dipole moments (TDM), Einstein coefficients (Aυ′υ″), Franck–Condon factors (qυ′υ″) and radiative lifetime (τυ′) for B1Σ+−X1Σ+ systems are predicted.

Structural, Spectroscopic and Vibrational Properties for Low-Lying Electronic States of LiCl

A multireference configuration interaction (MRCI) study has been carried out on the LiCl molecule. The potential energy has been calculated over a wide range of internuclear separation for the 21 low-lying electronic states of the LiCl molecule dissociating into Li (2S, 2P, 3S)+Cl (2P). The (4)1Σ+, (3)1Π, 1–33Σ+, 1–33Π, 1,3Δ, 1,3Σ−, (5)1Σ+, (4)3Σ+, (4)1Π, (4)3Π excited states are studied for the first time in theory. Molecular spectroscopic constants (Re,De, ωe, ωeχe, Be and αe) have been derived for the 9 bound states (X1Σ+, (3)1Σ+, (2)3Σ+, 1,3Δ, 1,3Σ−, (4)1Π, (4)3Π) with a regular shape, and the spectroscopic constants of ground states X1Σ+ are in good agreement with available experimental and theoretical values. The relative differences between experimental values and our values for Re, De,ωe, ωeχe, Be and αe are 1.02%, 0.60%, 1.72%, 9.46%, 2.0%, and 0.75%, respectively. Moreover, vibrational levels of 9 bound states, which have not been investigated experimentally, are computed.

A multireference configuration interaction study of CuB and CuAl molecular constants and photoionization spectra

Accurate potential energy curves and molecular constants for the low-lying electronic states of CuX(y) (X = B, Al; y = 0, +1) were investigated using the complete active space self-consistent field/multireference configuration interaction (MRCI) methodology with aug-cc-pV5Z basis set. The photoionization spectra of CuX were computed, showing electron detachment in the region of far ultraviolet. The results complement the previous theoretical characterizations and the few experimental studies. A comparative analysis was carried out concerning the different choices of reference configuration state functions in the MRCI calculations with and without the contribution of scalar relativistic effects. The results obtained with a small reference set adequately constructed are competitive to those using a much larger number of configuration state functions, and also the scalar relativistic effects improve significantly the molecular constants in this kind of system containing a 3d metal atom.

Geometries, vibrational frequencies, and excitation energies of a series of fluorine-substituted carbenes, FCX (X = H, F, Cl, Br, and I): A high-level multireference configuration interaction study

High-level calculations using internally contracted multireference configuration interaction including Davidson correction (icMRCI+Q) method have been carried out for the ground singlet states, the first excited states, and the lowest triplet states of a series of fluorine-substituted carbenes FCX (X = H, F, Cl, Br, and I). Equilibrium geometries and vibrational frequencies of the three electronic states, adiabatic transition energy of the first excited singlet state, as well as the ground singlet—lowest triplet energy gap (S-T gap) of each of FCX carbenes have been obtained. Effects of the basis set of icMRCI+Q calculation on the geometries and energies have been investigated. In addition, various corrections, including the scalar relativistic effect, spin-orbit coupling, and core-valence correlation, have been studied in calculating the transition energies and the S-T gaps, especially for heavy-atom carbenes. This results have been compared with previous calculations using a variety of methods. Our icMRCI+Q results are in very good agreement with the high-resolution laser-based spectroscopic results where available. Some structure and spectroscopic constants of the fluorine-substituted carbenes which are void in the literature have been provided with consistent high-level calculations. © 2013 Wiley Periodicals, Inc.

Multireference configuration interaction study on spectroscopic properties of low-lying electronic states of As2 molecule

The potential energy curves (PECs) of four electronic states (X1Σg+, e3Δu, a3Σu−, and d3Πg) of an As2 molecule are investigated employing the complete active space self-consistent field (CASSCF) method followed by the valence internally contracted multireference configuration interaction (MRCI) approach in conjunction with the correlation-consistent aug-cc-pV5Z basis set. The effect on PECs by the relativistic correction is taken into account. The way to consider the relativistic correction is to employ the second-order Douglas-Kroll Hamiltonian approximation. The correction is made at the level of a cc-pV5Z basis set. The PECs of the electronic states involved are extrapolated to the complete basis set limit. With the PECs, the spectroscopic parameters (Te, Re, ωe, ωexe, ωeye, αe, βe, γe, and Be) of these electronic states are determined and compared in detail with those reported in the literature. Excellent agreement is found between the present results and the experimental data. The first 40 vibrational states are studied for each electronic state when the rotational quantum number J equals zero. In addition, the vibrational levels, inertial rotation and centrifugal distortion constants of d3Πg electronic state are reported which are in excellent agreement with the available measurements. Comparison with the experimental data shows that the present results are both reliable and accurate.