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.     

The ground and low-lying excited potential curves of SO

A method is presented for approximating the effect of core electrons by a pseudopotential which is an extension of one previously presented by Dixon and Hugo. The pseudopotential is constructed in a fully ab initio manner from atomic SCF calculations. It is non-local in both radial and angular coordinates, but its matrix elements are none the less easy to evaluate. The method has been implemented within a multi-structure valence-bond framework. The approximations arising from the use of finite basis sets, both for the pseudopotential and for the valence wavefunction, inevitably lead to errors in calculated energies. However, these errors are largely atomic in origin. Thus, in addition to ab initio calculations we also use empirical atomsin-molecules corrections to minimize both basis set errors and atomic correlation errors. These method are applied to potential curves for 21 electronic states of the SO molecule. Comparison is made of the curves calculated using the ab initio multi-structure valence-bond method without and with the atoms-in-molecules corrections. The potential energy curves of three, previously unobserved, bound electronic states of SO are calculated. We estimate that these states, ¹Σ^-, ³Δ and ³Σ⁺, lie in the region of 3·2 to 3·4 eV above the ground state.     

Ab initio study of the feasibility of laser cooling of ScO molecule

ABSTRACT

Using ab initio quantum chemistry method, the feasibility of laser cooling ScO was investigated. The ground state Χ²Σ⁺ and low-lying excited states Α²Π, Α′²Δ are calculated at the multi-reference configuration interaction (MRCI) level of theory. The calculated spectroscopic constants are in good agreement with available theoretical and experimental results. At the MRCI level of theory with Davidson correction, the permanent dipole moments of the Χ²Σ⁺ and Α²Π states of ScO are also calculated. The highly diagonally distributed Franck–Condon factors and shorter radiative lifetime for the Α²Π→Χ²Σ⁺ transition are calculated with the corresponding potential energy curves and transition dipole moment. Although there is an intermediate state Α′²Δ, the loss will be dominated by branching to the intermediate electronic state Α′²Δ at a level of η < 1.4 × 10^?4. These results demonstrate the probability of laser cooling of ScO, and we provide a promising laser-cooling scheme for ScO molecule.     

Rydberg states of the HeH+ ion calculated by ab initio methods: potential energies and effective principal quantum numbers

More than 80 excited electronic states of the hydrohelium ion HeH⁺ of ^{1, 3}Σ⁺, ^{1, 3}Π, ^{1, 3}Δ, ^{1, 3}Φ and ^{1, 3}Γ symmetry have been calculated ab initio up to n = 6 for internuclear distances ranging from 0.5 to 100 bohr. The computations involve a configuration interaction (CI) treatment based on a home-made suite of programs that uses special basis sets designed for the representation of molecular Rydberg states. The results are compared with previous computations where these are available (up to n = 4), and it is found that except for the very lowest excited states, the present energies are consistently lower than those obtained previously, with an average lowering corresponding to several hundred cm^?1. It is shown that with the exception of its ground state, HeH⁺ is an effective one-electron system having an overall electronic structure similar to H⁺₂. The interaction of the excited electron with the He⁺ 1s core electron causes small singlet–triplet splittings to appear and ?-mixing interactions to occur, that are not present in H⁺₂.     

Electron scattering with open-shell molecules: R-matrix method

Many molecules with an even number of electrons belong to open-shell systems due to π ² ground state electronic configuration. This configuration gives rise to three low-lying states X ³ Σ ⁻, a Δ ¹ and b ¹ Σ ⁺. The inclusion of these target states in a trial wave function of the entire scattering system have important implications in the resonances that may be detected in these open-shell molecules. Various molecules like O₂, PX (X = H and halogens), SO, Si₂, BF have π ² ground state configuration. The R-matrix method is a well established ab initio formalism to calculate differential, integral and momentum-transfer cross sections for the elastic scattering of electrons by molecules. We have calculated these cross sections for PH and SO molecules in the incident electron energy range 0–10 eV. The results are obtained by using the R-matrix method in which the closecoupling expansion of the wave function of the scattering system includes only the ground state. This target state is described by configuration interaction wave function that includes correlation effects. The cross section for electron impact on PH and SO are presented.     

Fourier transform emission spectroscopy and ab initio calculations on WO

Emission spectra of WO have been observed in the 4000-35 000 cm⁻¹ region using a Fourier transform spectrometer. Molecules were produced by exciting a mixture of WCl₆ vapor and He in a microwave discharge lamp. A ³Σ⁻ state has been assigned as the ground state of WO based on a rotational analysis of the observed bands and ab initio calculations. After rotational analysis, a majority of strong bands have been classified into three groups. Most of the transitions belonging to the first group have an Ω = 0⁺ state as the lower state while the bands in the second group have an Ω′′ = 1 state as the lower state. These two lower states have been assigned as X0⁺ and X1 spin components of the X³Σ⁻ ground state of WO. The third group consists of additional bands interconnected by common vibrational levels involving some very low-lying states. The spectroscopic properties of the low-lying electronic states have been predicted from ab initio calculations. The details of the rotational analysis are presented and an attempt has been made to explain the experimental observations in the light of the ab initio results.     

The electronic structure of the low lying sextet and quartet states of CrF and CrCl

The electronic structure of CrF and CrCl in X ⁶Σ⁺, ⁶Π, ⁶Δ, A⁶Σ⁺, ⁴Σ⁺, ⁴Π, and ⁴Δ states that correlate with the low lying ⁶S, ⁶D, and ⁴D states of Cr⁺ have been studied, using large atomic natural orbital (ANO) basis sets and a variety of ab initio methods, including multi-reference configuration interaction (MRCI) and coupled cluster with perturbative triples (RCCSD(T)). We include scalar relativistic effects perturbatively and also explore the consequence of correlating the 3s and 3p electrons on the transition metal. We report T _e, R _e,ω_e, as well as dipole moments, bond energies, and charge distributions and compare with the available experimental data as well as previous theoretical results.     

Theoretical study of the TiC molecule: clarification of the ground state

The electronic structure of the TiC molecule was examined using three types of multi-reference single- and double-excitation configuration interaction (MRSDCI) calculations with highly extended basis sets. Multi-reference coupled pair approximation (MRCPA) was applied after the MRSDCI calculations that included core—valence and core—core correlation in addition to the valence correlation (v-c-c CI). From the results of the most accurate calculation with MRCPA (v-c-c CPA) it was concluded that a ¹Σ⁺ state is the ground state, despite previous calculations that suggested a ³Σ⁺ state with a ¹Σ⁺ state lying only slightly above it. The ¹Σ⁺ state is more highly correlated than the ³Σ⁺ state, and it was found that use of a size-consistent method is necessary to predict the relative stability accurately. The study evaluated the spectroscopic constants and considered the effect of the core (Ti 3p) correlation on these parameters. By taking the core correlation effect into account, the estimation of the dissociation energy (D _e) was improved dramatically; D _e obtained through v-c-c CPA was 4.457eV for the ¹Σ⁺ ground state, which agrees well with the experimental value (the latest being 4.35 ± 0.31 eV).     

BH分子基态和激发态解析势能函数和光谱性质

运用多参考组态相互作用的方法和Dunning’s相关调和基函数并含扩散基的大基组aug-cc-pV5Z,获得了BH分子基态（X¹Σ⁺）和6个电子激发态（a³П, A¹П, B¹Σ⁺, b³Σ⁺, b³ 关键词： 势能曲线 解析势能函数 多参考组态相互作用方法 光谱常数     

Electronic and vibrational spectroscopy of the low-lying states of potassium mono-sulphide KS,and comparison in the series of MS (M = Li,Na, K,Rb, Cs)

ABSTRACT

Potential energy curves (PECs) of the lowest electronic states of the potassium mono-sulphide KS have been determined with highly correlated ab initio calculations, using internally contracted multi reference interaction configuration methods including Davidson correction (MRCI?+?Q) with and without considering spin-orbit effects. For the three low-lying bound states, we report a set of spectroscopic parameters including equilibrium distances, dissociation energies, vibrational and rotational constants. The effects of spin-orbit-induced changes on these parameters are also discussed. An analysis of the properties of the three bound states, X²Π, 1²Σ⁺ and 2²Π, illustrates the common characteristics of the whole series of compounds in the MS family (M?=?Li, Na, K, Rb, Cs). Indeed, the shapes of the PECs of these bound states are strongly affected by the interactions between the two ionic states, ²Σ⁺ and ²Π, correlating at large internuclear separations (R_MS) to the first ionic dissociation limit [M⁺?+?S^?] and the electronic states correlating to the three/four lowest dissociation limits. The spectroscopy of these low-lying electronic states and the lifetime of their vibrational levels are thus affected by the spin-orbit interactions which are mainly related to the S atom and consequently common to all alkali-metal mono-sulphides.     

Feasibility study of laser cooling of InF molecule

By employing ab initio quantum chemistry method, we investigate the feasibility of laser cooling InF molecule. Four low-lying electronic states (X¹Σ⁺, C¹Π, ³Π and 2³Π) of InF have been calculated using the multi-reference configuration interaction (MRCI) method. The spin-orbit coupling effects are also taken into account in the electronic structure computation at the MRCI level. The highly diagonal Franck-Condon factors for C¹Π → X¹Σ⁺ transitions are estimated. The radiative lifetime of the C¹Π (v′ = 0) state is about 2.22 ns, which is found to be enough short for rapid laser cooling. Though the cooling wavelength of InF is located in the short-wavelength ultraviolet light (UVC), a frequency quadrupled Ti: sapphire laser (189–235 nm) could be capable of generating laser transition wavelength of InF. Furthermore, the C¹Π → X¹Σ⁺ transitions perhaps can be followed by the B³Π₁ → X¹Σ⁺_0⁺ transitions to attain a lower Doppler temperature. Meanwhile, for achieving quasi-closed transition cycle of InF molecule, we investigate the hyperfine structure of the lowest state X¹Σ⁺. Overall, the present results indicate the possibility of laser cooling InF molecules.     

The theoretical character of the X^1∑^＋ and A^1∑^＋ states of ScN

This paper calculates the potential energy curves (PECs) of the ground state (X 1 Σ + ) and excited state (A 1 Σ + ) of ScN molecule by multireference configuration interaction method. The correct character of the PECs has been gripped while they had been improperly reported in the literature. Based on the PECs, the spectroscopic parameters and vibrational energy levels are determined, and compared with experimental data and other theoretical works available at the present.     

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.     

Ab initio prediction of the infrared absorption spectrum of the C2Br radical

The first three electronic states of the C₂Br radical, correlating at linear geometries with ²Σ⁺ and ²Π states, have been studied ab initio, using Multi Reference Configuration Interaction techniques. The electronic ground state is found to have a bent equilibrium geometry, RCC=1.2621Å, R _CBr=1.7967Å, ∠ CCBr=156.1°, with a very low barrier to linearity. Similarly to the valence isoelectronic radicals C₂F and C₂Cl, this anomalous behaviour is attributed to a strong three-state non-adiabatic electronic interaction. The Σ ,Π_1/2,Π_3/2 vibronic energy levels and their absolute infrared absorption intensities at a temperature of 5 K have been calculated for the ¹² C¹² C⁷⁹Br isotopomer, to an upper limit of 2000 cm^?1, using ab initio diabatic potential energy and dipole moment surfaces and a recently developed variational method.     

BiH(D,T)分子基态结构与势能函数

采用量子力学从头计算方法,运用单双取代二次组态相互作用和单双（三）取代二次组态相互作用并结合LanL2DZ基组,计算优化了BiH（D,T）分子基态X³Σ的结构和离解能.并采用最小二乘法拟合改进的Murrell-Sorbie 函数得到了相应的势能函数.计算得到的光谱常数与实验光谱数据符合很好. 关键词： BiH（D T）分子基态 势能函数 Murrell-Sorbie函数     

GaH(D,T)分子基态结构与势能函数

采用量子力学从头计算方法,运用单双取代二次组态相互作用和单双（三）取代二次组态相互作用并考虑基组6-311++G（3df,3pd）计算优化了GaH（D,T）分子基态X¹Σ⁺的结构和离解能.并采用最小二乘法拟合改进的Murrell-Sorbie函数得到了相应的势能函数.计算得到的光谱常数与实验光谱数据符合得很好. 关键词： GaH（D T）分子基态 势能函数 Murrell-Sorbie函数     

Electronic states and spectroscopic properties of MgH in absence and presence of spin–orbit coupling − a configuration interaction study

Electronic spectrum of astrophysically important molecule magnesium hydride (MgH) has been studied using configuration interaction methodology excluding and including spin–orbit coupling. Potential energy curves of several spin-independent (Λ?S) electronic states have been constructed and spectroscopic constants of low-lying bound Λ?S states within 8.2 eV of term energy are reported in the first stage of calculations. The X²Σ⁺ is identified as the ground state in the Λ?S level. In the subsequent stage, the spin–orbit interaction has been incorporated and its effects on the potential energy curves and spectroscopic features of different electronic states of the species have been investigated. The X²Σ⁺_1/2 is identified as the spin–orbit (Ω) ground state of the species. Transition moments of several dipole-allowed transitions are computed in both the stages and radiative lifetimes of the corresponding excited states are computed. Electric dipole moments (µ) for a number of low-lying bound Λ?S states as well as several low-lying Ω-states are also calculated in the present study.     

All electron ab initio investigations of the electronic states of the FeC molecule

The low-lying electronic states of the molecule FeC have been investigated by performing all electron ab initio multi-configuration self-consistent-field (CASSCF) and multi reference configuration interaction (MRCI) calculations. The relativistic corrections for the one-electron Darwin contact term and the relativistic mass-velocity correction have been determined in perturbation calculations. The electronic structure of the FeC molecule is interpreted as antiferromagnetic couplings of the localized angular momenta of the ions and resulting in a triple bond in the valence bond sense. The electronic ground state is confirmed as being . The spectroscopic constants for the ground state and eleven excited states have been derived from the results of the MRCI calculations. The spectroscopic constants for the ground state have been determined as and ,and for the low-lying state as and . The values for the ground state agree well with the available experimental data. The FeC molecule is polar with charge transfer from Fe to C. The dipole moment has been determined as in the ground state and as 1.51 D in the state. From the results of the MRCI calculations the dissociation energy, , is determined as 2.79 eV, and D₀ as 2.74 eV. Received: 2 October 1998 / Received in final form: 30 March 1999     

Pt-Re small cluster interaction with H2

The interaction of small Pt-Re clusters with H₂ is reported here through ab initio multicon-figuration self-consistent field (MC-SCF) calculations, plus extensive multireference configuration interaction (MR-CI), variational and perturbative calculations. These calculations provide a cluster model for the activation of hydrogen by Pt-Re bimetallic catalysts. It was found that the ⁶S(5d⁵6s²) Re atom ground state needs an important activation to induce very weak capture of separated hydrogen atoms, whereas in the lowest excited states the activation energies are small or zero, with a very reasonable depth of well. The four lowest states of Pt-Re were found to be ⁴ Σ⁺, ⁶Π_yz, ^{Σ +} and ⁶Π_xz. Pt-Re interaction with H₂ has been studied from both metal ‘sides’. It was established that Pt-Re with the platinum side in the ground electronic ⁴Σ⁺ state and in the lowest ⁶Π⁺ excited states is able to capture H₂ molecules without activation, whereas in the ⁶Π_yz and ⁶Π_xz excited states there is no capture. The rhenium side of Pt-Re in its four lowest states considered cannot capture the H₂ molecule. The interaction of Pt₂-Re with H₂ was studied also. For the ground ²B₂ electronic state and the low lying ²A₁ electronic state the platinum moiety can spontaneously capture and break H₂. The rhenium side of Pt₂-Re(B₂), however, can capture H₂ only after surmounting a small barrier, and the excited Pt₂-Re(²A₁) can spontaneously capture H₂. For Pt₂-Re in its low lying ⁴A₁ electronic state both metal sides capture and break H₂ after surmounting a small barrier.