Contracted Multireference Configuration Interaction Research Articles

Spectroscopic properties of BCl (X1Σ+, a3Π, A1Π) molecule

The X1Σ+, a3Π and A1Π states of BCl molecule are studied using the highly accurate valence internally contracted multireference configuration interaction approach including the Davidson modification. The Dunning's correlation-consistent basis sets, aug-cc-pV6Z and aug-cc-pV5Z, are used in the study. To obtain more reliable results, the potential energy curves (PECs) of three electronic states are extrapolated to the complete basis set limit by the two-point total-energy extrapolation scheme. The effects of the core-valence correlation and relativistic corrections on the PECs are taken into account. By fitting these PECs, the spectroscopic parameters (Te, Re, ωe, ωexe, Be, αe and De) of the X1Σ+, a3Π and A1Π states of BCl are determined. These parameter values coincide with the experimental results. In addition, the whole vibrational states for X1Σ+, a3Π and A1Π states at J =0 (J is the rotational quantum number) are determined by numerically solving the radical Schrödinger equation of the nuclear motion of diatomic molecules. For each vibrational state, the vibrational level and inertial rotation constants are obtained, which are in excellent accordance with the experimental results. With the potential energy curves obtained at MRCI+Q/56+CV+DK level and the MRCI wave functions, the Franck-Condon factors, radiative lifetime of transition from a3Π and A1Π to the ground state are computed.

Ab initio MRCI + Q study on potential energy curves and spectroscopic parameters of low-lying electronic states of CS+

Carbon monosulfide molecular ion (CS+), which plays an important role in various research fields, has long been attracting much interest. Because of the unstable and transient nature of CS+, its electronic states have not been well investigated. In this paper, the electronic states of CS+ are studied by employing the internally contracted multireference configuration interaction method, and taking into account relativistic effects (scalar plus spin—orbit coupling). The spin—orbit coupling effects are considered via the state-interacting method with the full Breit—Pauli Hamiltonian. The potential energy curves of 18 Λ—S states correlated with the two lowest dissociation limits of CS+ molecular ion are calculated, and those of 10 lowest Ω states generated from the 6 lowest Λ—S states are also worked out. The spectroscopic constants of the bound states are evaluated, and they are in good agreement with available experimental results and theoretical values. With the aid of analysis of Λ—S composition of Ω states at different bond lengths, the avoided crossing phenomena in the electronic states of CS+ are illuminated. Finally, the single ionization spectra of CS (X1Σ+) populating the CS+(X2Σ+1/2, A2Π3/2, A2Π1/2, and B2Σ+1/2) states are simulated. The vertical ionization potentials for X2Σ+1/2, A2Π3/2, A2Π1/2, and B2Σ+1/2 states are calculated to be 11.257, 12.787, 12.827, and 15.860 eV, respectively, which are accurate compared with previous experimental results, within an error margin of 0.08 eV∼0.2 eV.

Accurate calculations on 12 Λ–S and 28 Ω states of BN+ cation: Potential energy curves, spectroscopic parameters and spin–orbit coupling

The potential energy curves (PECs) of 28Ω states generated from the 12 states (X4Σ−, 12Π, 12Σ−, 12Δ, 12Σ+, 22Π, A4Π, B4Σ−, 32Π, 16Σ−, 22Σ− and 16Π) of the BN+ cation are studied for the first time for internuclear separations from about 0.1 to 1.0nm using an ab initio quantum chemical method. All the Λ–S states correlate to the first four dissociation channels. The 16Σ−, 32Π and A4Π states are found to be the inverted ones. The 12Σ+, 22Π, 32Π and 22Σ− states are found to possess the double well. The PECs are calculated by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson correction. Core–valence correlation correction is included by a cc-pCV5Z basis set. Scalar relativistic correction is calculated by the third-order Douglas-Kroll Hamiltonian approximation at the level of a cc-pV5Z basis set. The convergent behavior of present calculations is discussed with respect to the basis set and level of theory. The spin–orbit coupling is accounted for by the state interaction approach with the Breit–Pauli Hamiltonian using the all-electron cc-pCV5Z basis set. All the PECs are extrapolated to the complete basis set limit. The spectroscopic parameters are obtained, and the vibrational properties of 12Σ+, 22Π, 32Π and 22Σ− states are evaluated. Analyses demonstrate that the spectroscopic parameters reported here can be expected to be reliably predicted ones. The conclusion is gained that the effect of spin–orbit coupling on the spectroscopic parameters are not obvious almost for all the Λ–S states involved in the present paper.

Theoretical study on thirteen Λ-S states of the SiN radical: potential energy curves, spectroscopic parameters and spin-orbit couplings

The spectroscopic properties of nineteen Ω states generated from the eight Λ-S bound states of SiN radicals are studied in detail using an ab initio quantum chemical method for the first time. The potential energy curves (PECs) are calculated by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification (icMRCI+Q). The spin-orbit coupling is accounted for by the Breit-Pauli Hamiltonian using the cc-pCVTZ basis set. Scalar relativistic correction calculations are made using the third-order Douglas-Kroll Hamiltonian approximation at the level of a cc-pV5Z basis set. Core-valence correlation corrections are included with a cc-pCVTZ basis set. All the PECs are extrapolated to the complete basis set limit. The convergent behavior is discussed with respect to the basis set and level of theory. The effect of core-electron correlations on the energy splitting is studied. Using the PECs, the spectroscopic parameters of twelve Λ-S and nineteen Ω bound states are evaluated. At the icMRCI+Q/CV+DK+56 theoretical level, the spin-orbit coupling splitting energy of the A 2 ΠΛ-S state is found to be 88.45 cm-1, which agrees well with the recent measurements of 89.09 cm-1. Moreover, other spectroscopic parameters are also in fair agreement with their respective measurements. The effect of SO coupling on the vibrational levels is discussed. It shows that the spectroscopic parameters of the twelve Λ-S and nineteen Ω bound states reported here can be expected to be reliably predicted ones.

Extensive calculations on 12 Λ-S and 27 Ω states of PCl+ cation including spin–orbit coupling

The potential energy curves (PECs) of 27 Ω states generated from the 12 Λ-S states (X2Π, A2Π, 14Π, 24Π, 12Σ−, 22Σ−, 14Σ−, 24Σ−, 12Σ+, 14Σ+, 12Δ and 14Δ) of PCl+ cation are studied for the first time for internuclear separations from about 0.10 to 1.10nm using an ab initio quantum chemical method. All the 12 Λ-S states correlate to the first dissociation channel of PCl+ cation. Of these Λ-S states, the 24Π is found to be the repulsive one. The 14Σ+, 12Δ and 14Δ are found to be the inverted ones. And the 12Δ is found to possess the double wells. The PECs are calculated by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson correction in combination with the correlation-consistent basis sets, aug-cc-pV(n+d)Z. The effect of core–valence correlation and scalar relativistic corrections on the spectroscopic parameters is briefly discussed. Scalar relativistic corrections are included by the third-order Douglas–Kroll Hamiltonian approximation at the level of a cc-pV5Z basis set. Core–valence correlation corrections are included with a cc-pCVTZ basis set. The convergent behavior of present calculations is discussed with respect to the basis set and level of theory. The spin–orbit coupling is accounted for by the state interaction method with the Breit–Pauli Hamiltonian using the all-electron cc-pCVTZ basis set. All the PECs are extrapolated to the complete basis set limit. The spectroscopic parameters are evaluated for the 11 Λ-S bound states and for the 23 Ω bound states, and are compared with available experimental and other theoretical results. Fair agreement has been found between the present spectroscopic parameters and the measurements. The energy splitting in the X2Π Λ-S state is calculated to be 346.11cm−1, close to the estimated measurements of 370cm−1. It demonstrates that the spectroscopic parameters reported here can be expected to be reliably predicted ones.

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.

On the theoretical description of nuclear quadrupole coupling in Π states of small molecules

Axial (eQq0) and perpendicular (eQq2) nuclear quadrupole coupling constants were evaluated from the electric field gradient at the quadrupolar nuclei (7Li, 14N, 17O, 33S, 35Cl) in diatomic (LiO, CN, NH+, NH, N2+, NO, OH, HCl+, CCl, OCl, NS) and polyatomic (C2N, C4N, NCO, N3) Π states. For diatomics the nuclear quadrupole coupling constants (NQCCs) were determined as a function of the vibrational quantum number. The calculations were performed using the internally contracted multireference configuration interaction and single-configuration coupled-cluster approaches with large correlation-consistent basis sets. The overall quality of the wave functions was tested by comparing the calculated electric dipole moments and diatomic spectroscopic constants with external data. The calculated NQCCs were discussed and compared with previous experimental and theoretical studies.

Accurate calculations on 9 Λ-S and 28 Ω states of NSe radical in the gas phase: Potential energy curves, spectroscopic parameters and spin–orbit couplings

The potential energy curves (PECs) of 28 Ω states generated from 9 Λ-S states (X2Π, 14Π, 16Π, 12Σ+, 14Σ+, 16Σ+, 14Σ-, 24Π and 14Δ) are studied for the first time using an ab initio quantum chemical method. All the 9 Λ-S states correlate to the first two dissociation limits, N(4Su)+Se(3Pg) and N(4Su)+Se(3Dg), of NSe radical. Of these Λ-S states, the 16Σ+, 14Σ+, 16Π, 24Π and 14Δ are found to be rather weakly bound states. The 12Σ+ is found to be unstable and has double wells. And the 16Σ+, 14Σ+, 14Π and 16Π are found to be the inverted ones with the SO coupling included. The PEC calculations are made by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification. The spin–orbit coupling is accounted for by the state interaction approach with the Breit-Pauli Hamiltonian. The convergence of the present calculations is discussed with respect to the basis set and the level of theory. Core-valence correlation corrections are included with a cc-pCVTZ basis set. Scalar relativistic corrections are calculated by the third-order Douglas-Kroll Hamiltonian approximation at the level of a cc-pV5Z basis set. All the PECs are extrapolated to the complete basis set limit. The variation with internuclear separation of spin–orbit coupling constants is discussed in brief for some Λ-S states with one shallow well on each PEC. The spectroscopic parameters of 9 Λ-S and 28 Ω states are determined by fitting the first ten vibrational levels whenever available, which are calculated by solving the rovibrational Schrödinger equation with Numerov’s method. The splitting energy in the X2Π Λ-S state is determined to be about 864.92cm−1, which agrees favorably with the measurements of 891.80cm−1. Moreover, other spectroscopic parameters of Λ-S and Ω states involved here are also in fair agreement with available measurements. It demonstrates that the spectroscopic parameters reported here can be expected to be reliable predicted ones.

Accurate theoretical study on 18 Λ-S and 50 Ω states of CS in the gas phase: Potential energy curves, spectroscopic parameters, and spin-orbit coupling

The potential energy curves (PECs) of 50 Ω states generated from the 18 Λ-S states are studied for the first time using the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification. All the 18 Λ-S states correlate to the first dissociation limit, C((3)Pg) + S((3)Pg), of CS molecule, of which only the 2(5)Π is repulsive and the A(1)Π, A(') (1)Σ(+), and 2(3)Σ(+) possess double wells. The spin-orbit (SO) coupling is accounted for by the state interaction approach with the Breit-Pauli Hamiltonian. Core-valence correlation correction is taken into account with an all-electron cc-pCV5Z basis set. Scalar relativistic correction calculations are made by the third-order Douglas-Kroll Hamiltonian approximation at the level of a cc-pVTZ basis set. All the PECs are extrapolated to the complete basis set limit. The a(') (3)Σ(+), e(3)Σ(-), 1(5)Σ(+), 1(5)Π, and d(3)Δ are found to be the inverted Λ-S states with the SO coupling included. The spectroscopic parameters of 17 Λ-S and 41 Ω bound states are evaluated. The comparisons between the present results and available measurements are performed, and excellent agreement has been found. It shows that the spectroscopic results reported here can be expected to be reliable predicted ones.

Ab Initio MRCI+Q Study on Low-Lying States of CS Including Spin–Orbit Coupling

Carbon monosulfide (CS), which plays an important role in a variety of research fields, has long received considerable interest. Due to its transient nature and large state density, the electronic states of CS have not been well understood, especially the interactions between different states. In this paper, we performed a detail ab initio study on the low-lying electronic states of CS by means of the internally contracted multireference configuration interaction method (including Davidson correction) with scalar relativistic correction using the Douglas-Kroll-Hess Hamiltonian. We focused on the spin-orbit coupling of the states via the state interaction method with the full Breit-Pauli Hamiltonian. The potential energy curves (PECs) of 18 Λ-S states correlated with the lowest dissociation limit of the CS molecule were calculated, as well as those of 50 Ω states generated from the Λ-S states. The spectroscopic constants of the bound states were obtained, which are in good agreement with previous available experimental and theoretical results. The state perturbations of the a(3)Π and A(1)Π states with other low-lying electronic states are discussed in detail, based on the calculated spin-orbit matrix as well as the PECs of the Ω states. Avoided crossing in the states of CS was indicated when spin-orbit coupling was taken into account. Finally, the allowed transition dipole moments as well as the lifetimes of the five lowest vibrational states of the A(1)Π1, A'(1)Σ(+)0(+) and a(3)Πi states were obtained.

Extensive ab initio study of the electronic states of S2 molecule including spin-orbit coupling

The potential energy curves (PECs) of 15 Λ-S states and 24 Ω states generated from the 13 Λ-S bound states of the S2 molecule are investigated in detail using an ab initio quantum chemical method. The PECs are calculated for internuclear separations from 0.12 to 1.10 nm by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification (MRCI + Q). The spin-orbit (SO) coupling effect is accounted for by the Breit–Pauli Hamiltonian. To discuss the effect on the energy splitting by the core-electron correlations, the all-electron basis set, cc-pCVTZ with and without 2s2p correlations, is used for the SO coupling calculations of the A3 and B′3Πg Λ-S states since their measurements can be found in the literature. By comparison, the cc-pCVTZ basis set with 2s2p correlations is chosen for the SO coupling calculations of 13 Λ-S bound states. To improve the quality of PECs, core-valence correlation and scalar relativistic corrections are included. Scalar relativistic correction calculations are made using the third-order Douglas–Kroll Hamiltonian (DKH3) approximation at the level of a cc-pV5Z basis set. Core-valence correlation corrections are taken into account with a cc-pCVTZ basis set. The spectroscopic parameters of 13 Λ-S bound states and 24 Ω states are calculated. With the PECs obtained by the MRCI + Q/aug-cc-pV6Z + CV + DK + SO calculations, the SO coupling splitting energies are 379.25 cm−1 between the A′3 and A′2 Ω state, 83.40 cm−1 between the A1 and A0− Ω state and 210.91 cm−1 between the B′2 and B′1 Ω state, which agree well with the corresponding measurements of 383, 77.51 and 209 cm−1, respectively. Moreover, other spectroscopic parameters are also in excellent agreement with the measurements. It demonstrates that the spectroscopic parameters of 24 Ω states reported here for the first time can be expected to be reliable predicted ones.

Potential Energy Curves, Spectroscopic Parameters, and Spin–Orbit Coupling: A Theoretical Study on 24 Λ-S and 54 Ω States of C2+ Cation

The potential energy curves (PECs) of 24 Λ-S states and 54 Ω states of the C2(+) cation are studied in detail using an ab initio quantum chemical method. All the PEC calculations are made for internuclear separations from 0.09 to 1.11 nm by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification (MRCI+Q). All the Λ-S states involved dissociate into the first dissociation limit, C((3)Pg) + C(+)((2)Pu), of C2(+) cation, of which only the 2(2)Σg(-) and 2(4)Σg(-) are repulsive. The spin-orbit (SO) coupling effect is accounted for by the Breit-Pauli Hamiltonian with an aug-cc-pCVTZ basis set. To improve the quality of PECs, core-valence correlation and scalar relativistic corrections are included. Core-valence correlation corrections are taken into account with an aug-cc-pCVTZ basis set. Scalar relativistic correction calculations are done by the third-order Douglas-Kroll Hamiltonian approximation with the cc-pVQZ basis set. All the PECs are extrapolated to the complete basis set limit. The convergence observations of present calculations are made, and the convergent behavior is discussed with respect to the basis set and level of theory. With the PECs obtained by the MRCI+Q/CV+DK+56 calculations, the spectroscopic parameters of 22 Λ-S bound states of C2(+) cation are evaluated by fitting the first ten vibrational levels, which are obtained by solving the rovibrational Schrödinger equation using Numerov's method. In addition, the spectroscopic parameters of 51 Ω bound states generated from these Λ-S bound states are also obtained. The spectroscopic parameters are compared with those reported in the literature. Excellent agreement with available measurements is found. It is expected that the spectroscopic parameters of Λ-S and Ω states reported here are reliable predicted ones.

Theoretical study on ten Λ–S states of [formula omitted] anion: Potential energy curves, spectroscopy and spin–orbit couplings

The potential energy curves (PECs) of seventeen Ω states generated from the ten Λ–S states of the Si2- anion are studied in detail using an ab initio quantum chemical method for the first time. The PECs are calculated for internuclear separations from 0.10 to 1.20nm by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification. The spin–orbit coupling is accounted for by the Breit–Pauli Hamiltonian. Core–valence correlation and scalar relativistic corrections are considered. Core–valence correlation corrections are included using an aug-cc-pCVTZ basis set. Scalar relativistic correction calculations are made with the third-order Douglas–Kroll Hamiltonian approximation at the level of a cc-pV5Z basis set. Obvious effect of core–valence correlation corrections on the PECs is observed, in particular for the two lowest 2Πu and 2Σg+ Λ–S states. All the PECs are extrapolated to the complete basis set limit. The lowest 2Πu Λ–S state is found to be the ground state of Si2- anion. The convergence observations of present calculations are made and the convergent behavior is discussed with respect to the basis set and level of theory. The effects of core-electron correlations on the energy splitting are studied by the all-electron aug-cc-pCVTZ basis set. Using these PECs, the spectroscopic parameters of Λ–S and Ω states involved are determined. The vibrational manifolds are evaluated for each Λ–S and Ω state of non-rotation Si2- anion. It shows that the spectroscopic parameters and molecular constants of ten Λ–S and seventeen Ω states reported here can be expected to be reliable predicted ones.

Potential energy curves, spectroscopic constants and spin-orbit coupling: A theoretical study on twelve triplet Λ-S states of Ge2 molecule

The potential energy curves (PECs) of twelve triplet Λ-S states and fourteen Ω states generated from five triplet Λ-S states of Ge2 molecule are calculated by an ab initio quantum chemical method. The PEC calculations are made for internuclear separations from 0.15 to 1.10 nm using the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification (MRCI + Q). The basis set used here is the aug-cc-pV5Z-pp. Convergent behavior is discussed and excellent convergence is observed with respect to the basis sets and level of theory. The PECs are extrapolated to the complete basis set limit by the total-energy extrapolation scheme. The spin-orbit coupling effect is accounted for by the Breit- Pauli Hamiltonian in combination with the aug-cc-pVTZ-pp basis set. With the PECs, the spectroscopic parameters are evaluated by fitting the first ten vibrational levels, which are obtained by solving the ro-vibrational Schrodinger equation using Numerov’s method. The spectroscopic results are compared with those reported in the literature. Excellent agreement with available measurements is found. The vibrational levels and inertial rotation constants are calculated for each Λ-S state and those of the first ten vibrational states are reported for the non-rotation Ge2 molecule. The spectroscopic parameters of the 13Σ + , 13Δ u , 23Σ + , 13Π g , 23Π g , 33Π g , 13Σ − , 23Σ − , 23Π u and 23Σ − Λ-S states and the vibrational levels and inertial rotation constants of all the Λ-S states determined by the MRCI + Q/Q5 calculations are expected to be reliable predicted ones.

MRCI+Q study on spectroscopic parameters and molecular constants of X1Σ+ and A1Π electronic states of the SiSe molecule

The potential energy curves (PECs) of X1Σ+ and A1Π electronic states of the SiSe molecule are calculated using the internally contracted multireference configuration interaction approach with the Davidson modification (MRCI+Q) with the correlation-consistent basis sets, aug-cc-pV5Z and aug-cc-pVQZ. In order to improve the quality of the PEC, the PEC is extrapolated to the complete basis set limit by the two-point total-energy extrapolation scheme; and the scalar relativistic correction is included. Scalar relativistic correction calculations are carried out using the third-order Douglas-Kroll Hamiltonian approximation at the level of a cc-pV5Z basis set. The spectroscopic parameters (Te, De, Re, ωe, ωexe, ωeye, Be and αe) of two states are calculated. The spectroscopic results are compared in detail with those reported in the previous literature. Excellent agreement is found between the present spectroscopic results and the experimental ones. Using the PECs obtained by the MRCI+Q/Q5+DK calculations, the Bυ and Dυ are calculated for each vibrational state of each electronic state, and their values of the first 30 vibrational states are reported for X1Σ+ and A1Π states of 28Si80Se molecule when J=0. Comparison with the measurements demonstrates that the present vibrational manifolds are both reliable and accurate. They should be good predictions for the future experimental or theoretical research.

Spectroscopic properties of AlC (X4∑-, B4∑-) molecule

The potential energy curves (PECs) of X4∑- and B4∑- states of the AlC molecule have been studied using highly accurate internally contracted multireference configuration interaction approach with the Davidson modification. The Dunning's correlation-consistent basis sets, aug-cc-pVnZ (n=D,T,Q,5,6) are used for the present study. To improve the quality of PECs, core-valence correlation and scalar relativistic corrections are considered. Core-valence correlation corrections are calculated with an aug-cc- pCVTZ basis set. Scalar relativistic correction calcualtions are made using the third-order Douglas-Kroll Hamiltonian approximation at the level of a cc-pV5Z basis set. Obvious effect on the PECs by the core-valence correlation and relativistic corrections has been observed. All the PECs are extrapolated to the complete basis set limit. The convergence observations of present calculations are made and the convergent behavior is discussed with respect to the basis set. Using these PECs, the spectroscopic parameters (TeReωeωexeωeyeBe and αe) of the X4∑- and B4∑- states are determined and compared with those reported in the literature. The vibration manifolds are evaluated for each state of non-rotation AlC molecule by numerically solving the radial Schrödinger equation of nuclear motion. For each vibrational state, the vibrational level and inertial rotation constants are obtained, which are in excellent accordance with the experimental findings.

A study on spectroscopic parameters of X2+, A2 and B2+ low-lying electronic states of SiN radical

The potential energy curves (PECs) of X2+, A2 and B2+ low-lying electronic states of SiN radical are investigated using the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach combining the full valence complete active space self-consistent field method. In the present calculations, the basis sets used are correlation-consistent basis sets, aug-cc-pV6Z. The PECs determined by the MRCI calculations are corrected for size-extensivity errors by means of the Davidson modification (MRCI +Q). To obtain more reliable results, effects of the core-valence correlation and relativistic correction on the PECs are taken into account. The core-valence correlation correction is carried out with the cc-pCV5Z basis set The way to consider the relativistic correction is to use the second-order Douglas-Kroll Hamiltonian approximation, and the correction is performed at the level of cc-pV5Z basis set. With these PECs, the spectroscopic parameters are determined. A comparison with the experimental data shows that the present spectroscopic parameters are more accurate than the previous calculations.

State-to-State Photodissociation Dynamics of H2O in the B-band: Competition between Two Coexisting Nonadiabatic Pathways

The photodissociation of H2O in its B band is a prototype for nonadiabatic reaction dynamics. In addition to dissociation via the adiabatic pathway to the OH(Ã(2)Σ(+)) + H fragments, it also produces the OH(X̃(2)Π) + H fragments through two nonadiabatic pathways: the B̃ → X̃ transition via two conical intersections and the B̃ → Ã transition via a Renner-Teller pair. In this work, the state-to-state dissociation dynamics in all three channels are investigated with a full-dimensional quantum mechanical model using a set of coupled diabatic potential energy surfaces determined at the internally contracted multireference configuration interaction level with the aug-cc-pVQZ basis set. The inclusion of all relevant electronic states not only results in an improved agreement with the latest experimental data but also sheds valuable insights into the competition between the two coexisting nonadiabatic pathways.

A comparison of various approaches in internally contracted multireference configuration interaction: the carbon dimer as a test case

Various variational spaces deriving from different contraction schemes of the first-order interacting space have been examined and their performances have been tested on the carbon dimer. The contraction schemes stemming from the strongly contracted and from the partially contracted approach of the n-electron valence state perturbation theory have been analysed along with the mixed approach of Werner and Knowles as well as with the totally uncontracted approach where the full dimensionality of the first-order interacting space is taken into account. The results are very similar for all the spaces considered, despite their widely varying dimensionality. A comparison with the second-order and the third-order results of NEVPT is also reported.