Ab initio configuration interaction study on the electronic structure of the X2Σ+, B2Σ+ and 32Σ+ states of SiO+

The energy levels and electronic structure of the X²Σ⁺, B²Σ⁺ and 3²Σ⁺ states of SiO⁺ are studied using ab initio configuration interaction (CI) calculations at and around their equilibrium internuclear distances R _e. Spectroscopic constants and the vertical excitation energy from the SiO⁺ X²Σ⁺ state are predicted for the 3²Σ⁺ state. Based on the calculated CI wavefunctions, avoided crossings of the potential energy curve for the 3²Σ⁺ state and a near-degeneracy effect in the avoided crossing region are examined. The effects of the mixing of excited configuration state functions in the total electronic wavefunctions for the 1–3 ²Σ⁺ states are investigated by analysing correlation energies in terms of the contributions from classes of excited configurations. The importance of both the near-degeneracy effect and the correlation energy effect in describing correctly the electronic structure of the 3 ²Σ⁺ state in the neighbourhood of its R _e is discussed.     

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＋（X2Σ1/2＋, A2Π3/2, A2Π1/2, and B2Σ1/2＋） states are simulated. The vertical ionization potentials for X2Σ1/2＋, A2Π3/2, A2Π1/2, and B2Σ1/2＋ 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.     

Ab initio configuration interaction study of the ground and low-lying excited states of ZnCd

The multi-reference configuration interaction (MRCI) electronic energy calculations have been carried out on the ground state (X~1∑) as well as three low-lying excited states (~3E,~1∏,~3∏) of ZnCd dimer.Poten- tial energy curves (PECs) are therefore generated and fitted to the analytical potential energy functions (APEFs) using the Murrel-Sorbie (MS) potential function.Based on the PECs,the vibrational levels of each state are determined by solving Schr(?)dinger equation of nuclear motion,and corresponding spec- troscopic parameters are accurately calculated using the APEFs.The present values of spectroscopic parameters including equilibrium positions and dissociation energies are compared with other theoretical reports available at present.     

A comment on “Ab initio study: the potential energy curves and ro-vibrational spectrum of low-lying excited states of HCl+ cation”

Since the ²Π state in HCl⁺ is an inverted doublet, the energy of the ²Π_1/2 state is higher than the ²Π_3/2. Therefore, the larger value of intensity correspond to the transition of ²Π_3/2. We calculated the Einstein A coefficients and radiation lifetimes for the A²Σ⁺-X²Π transition. Our results are in good agreement with the experimental data and theoretical values. Then the ro-vibrational line intensities of the 1-0 band were calculated for the ²Π_3/2 and ²Π_1/2 states of HCl⁺. Employing the RKR potential, the predicted band origins for Δν=1-0 are 2569.3 and 2568.55 cm-1 for ²Π_3/2 and ²Π_1/2, respectively.     

Ab initio multi-reference configuration interaction of the low-lying states of the AsP molecule

Nine low-lying electronic states of the AsP molecule,includingΣ~ ,Π,andΔsymmetries with singlet, triplet,and quintet spin multiplicities,are studied using multi-reference configuration interaction method. The potential energy curves and the spectroscopic constants of these nine states are determined,and compared with the experimental observed data as well as other theoretical works available at present. Three quintet states are reported for the first time.Furthermore,the analytical potential energy functions of these states are fitted using Murrell-Sorbie function and least square fitting method.     

Ab initio potential surfaces for the à and states of HCN+

Ab initio configuration interaction calculations for the à ²Σ⁺ and states of HCN⁺ are presented. Minima occur at r _CH = 2·03 a ₀, r _CN = 2·25 a ₀ (à ²Σ⁺) and r _CH = 2·75 a₀, r _CN = 2·26 a ₀ (). The potential surface for the state has a local maximum as the hydrogen atom is pulled away from CN. The barrier height is calculated to be 0·27 eV.     

Ab initio electronic band structure study of III–VI layered semiconductors

We present a total energy study of the electronic properties of the rhombohedral γ-InSe, hexagonal ε-GaSe, and monoclinic GaTe layered compounds. The calculations have been done using the full potential linear augmented plane wave method, including spin-orbit interaction. The calculated valence bands of the three compounds compare well with angle resolved photoemission measurements and a discussion of the small discrepancies found has been given. The present calculations are also compared with recent and previous band structure calculations available in the literature for the three compounds. Finally, in order to improve the calculated band gap value we have used the recently proposed modified Becke-Johnson correction for the exchange-correlation potential.     

Accurate ab initio potential energy curves and spectroscopic properties of the low-lying electronic states of OH− and SH− molecular anions

ABSTRACT

Multireference configuration interaction method was used in order to generate accurate potential energy curves of the OH, SH, OH^? and SH^? electronic states correlating to the three lowest dissociation limits. These curves were used in addition with core–valence correlation and scalar relativistic corrections for the calculations of accurate spectroscopic constants of bound states, which generally are found in excellent agreement with best available experimental and theoretical values in the literature. The spin–orbit interactions between electronic states have been calculated for the cases in which the couplings were assumed to be responsible for perturbations and used to explain the predissociation of A²Σ⁺ state of OH and SH by dissociative states 1⁴Σ^?, 1²Σ^? and 1 ⁴Π. Dipole moment functions were also computed along internuclear distances and used to explain polarity of these molecules in different calculated electronic states. In addition, stability and metastability of electronic states (X ¹Σ⁺, A¹Π and a³Π) of OH^? and SH^? molecular anions have been studied relatively to curves of neutral parent electronic states. Finally, we have computed adiabatic electron affinity of OH and SH and these values have been found in very good agreement with the best experimental values and resort as among the best achieved values.     

Ab initio study of PN electronic states – a qualitative interpretation of the perturbation and predissociation effects on observed transitions

Valence and high electronic states of PN have been calculated with accurate quantum chemistry methods. The variety of theoretical methods used includes complete active space self-consistent field, multireference configuration interaction and the newly developed explicitly correlated coupled cluster methods. The large correlation-consistent atomic orbitals basis sets AVQZ, AV5Z and AV(5+d)Z are used for the potential energy curves calculations in the bonding and long-range regions. The spectroscopic constants (R_e, B_e, ω_e, ω_ex_e, α_e, D_e, T_e) and the vibrational levels of the bound valence states (X¹Σ⁺, A¹Π, a³Σ⁺, d ³Δ, e³Σ^?, C¹Σ^?, b³Π, D ¹Δ and E¹Σ⁺ and some higher bound states) are determined and compared with experimental findings when available. Significant spin–orbit interactions between triplet states and A¹Π and E¹Σ⁺ excited states are found near the crossing points of the potential energy curves and could explain predissociation phenomena and the perturbations of the vibrational levels experimentally observed for PN in their A¹Π and E¹Σ⁺ states.     

Comment on ‘Ab initio study: the potential energy curves and rovibrational spectrum of low-lying excited states of HCl+ cation’ (Molecular Physics, 114:19, 2817–2823)

In a recent article, Y. Liu, X. Cheng, H. Cheng, J. Cheng and X. Song [Mol. Phys., 114:19, 2817–2823] report on the calculation of potential energy curves and the derivation of spectroscopic constants and line intensities for the lowest energy electronic states of HCl⁺. There are several shortcomings in the article; the most notable being the incorrect ordering of the ²П_3/2 and ²П_1/2 state energies, erroneous selection rules, which lead to the absence of a Q-branch in their predicted spectra, incorrect Höln–London factors, and a missing appendix.     

Characterization of the [Xtilde] 1Σ+ à 3Π and à 1Π electronic states of BBO

The three lowest-lying electronic states, [Xtilde] ¹Σ⁺, à ³II and à ¹II, of the linear BBO molecule have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties including dipole moments, vibrational frequencies and associated infrared intensities, Renner parameters and energetics for the three states of BBO have been determined employing SCF, CISD, CCSD and CCSD(T) levels of theory and a wide range of basis sets. The ground state of BBO presents a degenerate real bending frequency, while the à ³II and à ¹II states show two distinct real bending frequencies due to the Renner-Teller interaction. The bending motion of the à ¹II state was analysed using the equation-of-motion (EOM)-CCSD and EOM-CC3 techniques in order to avoid possible variational collapse to a lower-lying state. The [Xtilde] ¹Σ⁺-à ³II separation was predicted to be T ₀ = 16.6 kcal mol^?1 (5800 cm^?1, 0.719 eV) at the cc-pVQZ CCSD(T) level of theory. With the cc-pVQZ EOM-CC3 method the [Xtilde] ¹Σ⁺-à ¹II splitting was predicted to be T ₀ = 48.0 kcal mol^?1 (16 800 cm^?1, 2.08 eV), which is in good agreement with the experimental value of T ⁰ = 46.6 kcal mol^?1 (16 300 cm^?1, 2.02 eV). The Renner parameters and averaged harmonic frequencies of the bending mode were determined to be ? = 0.184 and ω₂ = 363 cm^?1 for the à ³II state, and ? = 0.246 and ω₂ = 383cm^?1 for the à ¹II state. The theoretical [Xtilde] ¹Σ⁺ state harmonic B-B stretching frequency ω₃ = 636 cm^?1 is somewhat higher than the experimental estimate of 582 cm^?1 and the predicted à ¹II state harmonic B-B stretching frequency ω₃ = 861 cm^?1 is significantly higher than the experimental estimate of 440 cm^?1     

Potential energy curves and analytical potential energy functions of the metastable states of B2^＋＋

The multi-reference configuration interaction method and aug-cc-pvqz （AVQZ） have been used to calculate potential energy curves （PECs） of the singlet and triplet states of the riu and rig symmetry of B2＋＋. All of the four states （^l∏u, ^1∏g, ^3∏u and ^3∏g） are found to be metastable states, though the potential well of ^3∏u symmetry is very shallow. Based on the PECs, the analytical potential energy functions （APEFs） of these states have been fitted using the least square fitting method and two models of function. The spectroscopic parameters of each state are also calculated, and are compared with other investigations in the literature. The credibility and veracity of the two functions are evaluated. Some ideas to improve the fitting accuracy are presented. Also the vibrational levels for each state are predicted by solving the SchrSdinger equation of nuclear motion.     

Ab initio study on the ground states,phase stability,and mechanical properties of the Au–Pt system

For the equilibrium immiscible Au–Pt system, ground states are studied based on the results of the cluster expansion method combined with ab initio calculations. The obtained results show that there is no stable phase for the Au–Pt system at 0 K. The further obtained enthalpies of formation for hypothetical crystalline L1₂, D0₁₉, D0₃ structured Au₃Pt and AuPt₃, as well as L1₀ structured AuPt compounds also have positive values. Moreover, elastic constants are predicted from ab initio for the first time for the metastable L1₂ Au₃Pt, AuPt₃ and L1₀ AuPt compound. Finally, there is an imaginary phonon appearing in the obtained phonon spectra, implying an internal instability of the positions of the nuclear coordinates of the L1₂Au₃Pt compound.     

The theoretical study on the potential energy curves for X^1 ∑^＋, A^1 ⅡI and C^1 ∑^- states of SiO molecule

This paper applies the symmetry-aziapted-cluster/symmetry-adapted-cluster configuration-interaction （SAC/SACCI） method to optimize the structures for X^1∑^＋, A^1 Ⅱ and C^1 ∑^- states of SiO molecule with the basis sets D95＋＋, 6-311＋＋G and 6-311＋＋G^＊＊. Comparing the obtained results with the experiments, it gets the conclusion that the basis set 6-311＋＋G^＊＊ is most suitable for the optimal structure calculations of X^1.∑^＋, A^Ⅱ and C^1∑^- states of SiO molecule. The whole potential energy curves for these electronic states are further scanned by using SAC/6-311＋＋G^＊＊ method for the ground state and SAC-CI/6-311＋＋G^＊＊ method for the excited states, then use a least square method to fit Murrell~Sorbie functions, at last the spectroscopic constants and force constants are calculated, which are in good agreement with the experimental data.     

Fine structure of the close-lying A 2Π and B 2Σ+ states of BaH and BaD

Molecular parameters for the close-lying and strongly interacting A ²Π and B ²Σ states of BaH and BaD have been re-evaluated by means of a numerical matrix diagonalization procedure. The results obtained according to this exact method deviate considerably from the effective ones of previous investigations, particularly with respect to the A ²Π-B ²Σ⁺ interaction matrix elements which describe the large Λ-doubling and spin-splitting. The new values of the Λ-doubling and spin-splitting parameters are in excellent agreement with pure precession values for L = 2, and thus the present results form an interesting extension of the pure precession model which so far has been found applicable in a number of cases for which L equals one. The pure precession result L = 2 indicates that the outermost electron of the A ²Π and B ²Σ⁺ states must be a d-electron, and this requires a re-assignment of the configuration quantum numbers of these states. Strong local perturbations are observed in the rotational levels of the A ²Π state of both BaH and BaD, and the result L = 2 now yields a further confirmation of the previous assumption that a ²Δ state causes these perturbations. In the case of BaD the electronic + vibrational energy and the rotational constants (B_v , D_v ) of the perturbing level could be determined from the perturbed A ²Π term values, and in particular the value of the interaction matrix element leads to the conclusion that there is a A ²Π, v = 0 - ²Δ, v = 2 interaction. Finally the influence of the A ²Π - ²Δ, Δv = 0 interaction on the A ²Π and B ²Σ⁺ molecular parameters was investigated.     

Ab initio MRCI+Q study on the low-lying excited states of the PBr radical including spin–orbit coupling†

High-level ab initio calculations have been performed on the PBr radical by using multi-reference configuration interaction method plus Davidson correction (+Q) with correlation-consistent quadruple-ζ quality basis set. The potential energy curves (PECs) of the 22 Λ–S states of PBr have been obtained, most of which are reported for the first time. From the PECs of the bound states, the spectroscopic constants have been determined, in good agreement with the experimental results where available. Due to the large state density, there exhibits complicated interactions in the electronic excited states of PBr. The possible interactions by the spin–orbit coupling (SOC) effect have been discussed based on the evaluated R-dependent spin–orbit matrix elements. The 51 Ω states, generating from the 22 Λ–S states after taking SOC into account, have been computed. The Λ–S component analysis of the wavefunctions for the Ω states indicates the strong interaction of the Λ–S states especially at the avoided crossing points and near the dissociation limits. Finally, the transition dipole moments of several transitions arising from upper Ω states to the X₁0⁺ and X₂1 states and the corresponding radiative lifetimes have been studied. Our calculation results provide new information that should be valuable for further experimental studies on the electronic excited states of the PBr radical.     

Coupled ab initio potential energy surfaces for the two lowest 2A′ electronic states of the C2H molecule

Realistic two-valued potential energy surfaces for the reaction C(³P) + CH(X²Π) → C₂ + H have been constructed from a set of high level ab initio data describing the first two ²A′ electronic states of the C₂H system. These states have linear equilibrium configurations, known as the X ²Σ⁺ and A²Π states, and are coupled by a conical intersection. They lead to the formation of C₂(X¹Σ⁺ _g) and C₂(a³Π_u) considering an adiabatic dissociation process. The ab initio calculations are of the multireference configuration interaction variety and were carried out using a polarized triple-zeta basis set. Using the ab initio adiabatic energies and the matrix elements of the dipole moment, a 2 × 2 diabatic representation of the electronic Hamiltonian was built. Each element of this Hamiltonian matrix was expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree-Fock approximate correlation energy model (EHFACE). The analytical adiabatic potential energy surfaces are then obtained as the eigenvalues of this matrix, and display correctly the Σ/Π conical intersection. Moreover, the non-adiabatic couplings given by our analytical model are compared with the ab initio ones, and good qualitative agreement is observed.     

MRCI potential energy curves and analytical potential energy functions for the X^2∑^＋ and 2^∏ states of BO molecule

The potential energy curves （PECs） of BO molecule, including ∑^＋and ∏ symmetries with doublet spin multiplicities, are obtained employing multi-reference configuration interaction （MRCI） method and Dunning＇s correlation consistent basis sets. The analytical potential energy functions （APEFs） are fitted using the Murrell-Sorbie （MS） function and the least square method. Based on the PECs, the spectroscopic constants of the states have been determined and compared with the theoretical and experimental results available to affirm the accuracy and liability of the calculations. The root-mean-square （RMS） errors between the fitted results and the ab initio values are too little in comparison with the chemical accuracy （349.755 cm^-1）. It is shown that the present APEFs are accurate and can display the interaction between the atoms well. The present APEFs can be used to construct more complicated APEF or do some dynamic investigations.     

Ab initio study of the Ar–CS2(V1B2) intermolecular potential surface: effect of van der Waals interaction on the emission of CS2 molecule

In this work, the excited state intermolecular potential energy surface of the Ar–CS₂(V¹B₂) van der Waals complex was evaluated for the first time. The calculation of more than 4000 single-point interaction energies for the complex using an equation-of-motion coupled-cluster model with single and double substitutions level of theory with extended basis set involving bond functions has been performed. After fitting the interaction energies to analytical functions, the emission spectra of the Ar–CS₂(V¹B₂) complex related to the different stationary points on the potential energy surface were calculated. It was seen that the intensity and the position of the emission spectra are dependent on the orientation of the Ar atom around the bent excited CS₂ and the distance between two components. The information about the structural parameters of the complex related to the global minimum was obtained under the pseudodiatomic approximation with assistance of ab initio potential. The presented investigation could be useful for further theoretical and experimental studies of Ar–CS₂(V¹B₂) complex.