Ab initio study of the low-lying electronic states of the CH2NO2 radical

Ab initio electronic structures calculations are reported for the four low-lying electronic states X ²B₁, ²B₂, ²A₂, and ²A₁ of the CH₂NO₂ radical. The geometric parameters for the ground-state X ²B₁ are predicted by MRSDCI calculations with a double zeta plus polarization basis set. The vertical excitations energies for these electronic states are determined using MRSDCI /DZ +P calculations at the ground-state equilibrium geometry and in agreement with the recent experimental data obtained via PES of the CH₂NO anion. The oscillator strenghts and the radiative lifetimes for these electronic states and the spin properties for the ground state are calculated based on the MRSDCI wave functions, predicting results in good agreement with available experimental data. © 1994 John Wiley & Sons, Inc.     

Ab initio study of low-lying electronic states of the PF2 radical

The equilibrium geometries, excitation energies, force constants, and vibrational frequencies of the low-lying electronic states X²B₁, ²A₁, ²B₂, and ²A₂ of the PF₂ radical have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated geometry, force constants, and vibrational frequencies for the X²B₁ state are in good agreement with experimental data. The electronic transition moments, oscillator strengths for the ²A₁ → X²B₁ and ²A₂ → X²B₁ transitions, and radiative lifetimes for the ²A₁ and ²A₂ states are calculated based on the MRSDCI wave functions. © 1994 by John Wiley & Sons, Inc.     

The MRSDCI studies of four low-lying electronic states of the BF2 radical

The equilibrium geometries, excitation energies, force constants, and vibrational frequencies for four low-lying electronic states X ²A₁, ²B₁, ²B₂, and ²A₂ of the BF₂ radical have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated excitation energy for X²A₁ → ²B₁ is in agreement with available experimental data. The electronic transition dipole moments, oscillator strengths for the ²B₁ → X²A₁ and ²B₂ → X²A₁ transitions, radiative lifetimes for the ²B₁ and ²B₂ states, and the spin properties for the X²A₁ state are calculated based on the MRSDCI wave functions, predicting results in reasonable agreement with available experimental data. © 1993 John Wiley & Sons, Inc.     

MRSDCI studies of low-lying electronic states of the CF 2 + ion

Summary The equilibrium geometries, excitation energies, force constants and vibrational frequencies for four low-lying electronic statesX ² A ₁,² B ₁,² B ₂ and² A ₂ of the CF ₂ ⁺ ion have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated excitation energies for these states and vibrational frequencies for the ground state are in good agreement with experimental data via photoelectron spectroscopy of the CF₂ radical (carbene). The electronic transition dipole moments, oscillator strengths for the² B ₁ X ² A ₁ and² B ₂ X ² A ₁ transitions, radiative lifetimes for the² B ₁ and² B ₂ states and the spin properties for theX ² A ₁ state are calculated based on the MRSDCI wavefunctions.     

An ab initio theoretical study of the optical spectrum of CF2

The ab initio calculation methods have been used to calculate the spectral and electronic characteristics of difluorocarbene in the ground electronic state (¹A₁), the lowest-lying singlet (¹B₁) and triplet (³B₁) states. The optimized equilibrium geometries, rotational constants, harmonic vibrational frequencies and energy gaps, electronic charges, dipole moments of these states have been computed with different basis sets. The calculated vibrational frequency of ³B₁ state (v₂=522 cm^?1) and the energy separation (2.26 eV) between ³B₁ and ¹A₁ states are in good agreement with the experimental results (519 cm^?1, 2.46 eV respectively). According to the calculations the previous assignment of vibrational symmetries of ¹B₁ state was corrected, and some experimentally undetermined vibrational frequencies were predicted.     

A theoretical study on the ionization of tetrafluoroethylene with analysis of vibrational structure of the photoelectron spectra

Ab initio calculations have been performed to study on the molecular structures and the vibrational levels of the low-lying ionic states (²B_2u,²A_g,²B_2g,²B_3u,²A_u,²B_1g,²B_1u, and²B_3g) of tetrafluoroethylene. The equilibrium molecular structures and vibrational modes of these states are presented. The theoretical ionization intensity curves including the vibrational structures of the low-lying eight ionic states are also presented and compared with the photoelectron spectrum. Some new assignments of the photoelectron spectra are proposed.     

AGP Propagator calculations

The use of the antisymmetrized geminal power (AGP ) consistent reference state for RPA polarization propagator calculations is explored. Illustrative applications to the three lowest electronic states (³B₁, ¹A₁, and ¹B₁) of the methylene radical are reported with comparisons to other theoretical methods and experiment.     

Nonadiabatic coupling and diabatic electronic population dynamics on 11A2 and 11B1 states of ozone molecule

In this work, we examine nonadiabatic population dynamics for 1¹B₁ and 1¹A₂ states of ozone molecule (O₃). In O₃, two lowest singlet excited states, ¹A₂ and ¹B₁, can be coupled. Thus, population transfer between them occurs through the seam involving these two states. At any point of the seam (conical intersection), the Born-Oppenheimer approximation breaks down, and it is necessary to investigate nonadiabatic dynamics. We consider a linear vibronic coupling Hamiltonian model and evaluate vibronic coupling constant, diabatic frequencies for three modes of O₃, bilinear and quadratic coupling constants for diabatic potentials, displacements, and Huang-Rhys coupling constants using ab initio calculations. The electronic structure calculations have been performed at the multireference configuration interaction and complete active space with second-order perturbation theory with a full-valence complete active space self-consistent field methods and augmented Dunning's standard correlation-consistent-polarized quadruple zeta basis set to determine ab initio potential energy surfaces for the ground state and first two excited states of O₃, respectively. We have chosen active space comprising 18 electrons distributed over 12 active orbitals. Our calculations predict the linear vibronic coupling constant 0.123 eV. We have obtained the population on the 1¹B₁ and 1¹A₂ excited electronic states for the first 500 fs after photoexcitation.     

Theoretical study of the electronic spectra ofcis-1,3,5-hexatriene andcis-1,3-butadiene

Summary The electronic spectra forcis-1,3-butadiene andcis-1,3,5-hexatriene have been studied using multiconfiguration second-order perturbation theory (CASPT2) and extended ANO basis sets. The calculations comprise all singlet valence excited states below 8.0 eV, the first 3s, 3p, 3d Rydberg states, and the second 3s state. The four lowest triplet states were also studied. The resulting excitation energies forcis-hexatriene have been used in an assignment of the experimental spectrum, leading to a maximum deviation of 0.13 eV for the vertical transition energies. The calculations place the 1¹ B ₂ state 0.04 eV below the 2¹ A ₁ state. 16 excited states were studied incis-butadiene, using a CASPT2 optimized ground state geometry. The 1¹ B ₂ state was located at 5.58 eV, 0.46 eV below the 2¹ A ₁ state and 0.09 eV above the experimental value. No experimental assignments are available for the 15 other transitions. On leave from: Departmento de Quimica Física, Universidad de Valencia, Dr. Moliner 50, Burjassot, E-46100-Valencia, Spain     

Calculations on 1,8-naphthoquinone predict that the ground state of this diradical is a singlet

(12/12)CASPT2, (16/14)CASPT2, B3LYP, and CCSD(T) calculations have been carried out on 1,8-Naphthoquinone (1,8- NQ ), to predict the low-lying electronic states and their relative energies in this non-Kekulé quinone diradical. CASPT2 predicts a ¹A₁ ground state, with three other electronic states—³B₂, ³B₁, and ¹B₁—within about 10 kcal/mol of the ground state in energy. On the basis of the results of these calculations, it is predicted that NIPES experiments on 1,8- NQ ^•– will find that 1,8- NQ is a diradical with a singlet ground state. © 2018 Wiley Periodicals, Inc.     

A CAS study on S‐loss and O‐loss dissociation mechanisms of the SO2+ ion in the C,D, and E states

In the present work, we mainly study dissociation of the C ²B₁, D²A₁, and E²B₂ states of the SO₂⁺ ion using the complete active‐space self‐consistent field (CASSCF) and multiconfiguration second‐order perturbation theory (CASPT2) methods. We first performed CASPT2 potential energy curve (PEC) calculations for S‐ and O‐loss dissociation from the X, A, B, C, D, and E primarily ionization states and many quartet states. For studying S‐loss predissociation of the C, D, and E states by the quartet states to the first, second, and third S‐loss dissociation limits, the CASSCF minimum energy crossing point (MECP) calculations for the doublet/quartet state pairs were performed, and then the CASPT2 energies and CASSCF spin‐orbit couplings were calculated at the MECPs. Our calculations predict eight S‐loss predissociation processes (via MECPs and transition states) for the C, D, and E states and the energetics for these processes are reported. This study indicates that the C and D states can adiabatically dissociate to the first O‐loss dissociation limit. Our calculations (PEC and MECP) predict a predissociation process for the E state to the first O‐loss limit. Our calculations also predict that the E²B₂ state could dissociate to the first S‐ and O‐loss limits via the A²B₂ ← E²B₂ transition. On the basis of the 13 predicted processes, we discussed the S‐ and O‐loss dissociation mechanisms of the C, D, and E states proposed in the previous experimental studies. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Theoretical evidence for bound electronic excited states of ozone

Extensive configuration interaction calculations (up to 1532 spin eigenfunctions) have been carried out on ozone with both minimal and extended bases. Vertical and adiabatic excitation energies to 14 excited states are reported, including seven states with vertical excitation energies less than 4 eV. Our calculations indicate that in addition to the ground state there are four other states of ozone (³B₂, ³A₂, ¹A₂ and ³B₁) bound with respect to dissociation to ground state O₂ and O (by 0.4, 0.3, 0.1 and 0.0 eV, respectively). With such small bonding energies, the current results cannot be said to show definitively (except perhaps for ³B₂) these four states to be bound with respect to O₂ + O. However, the theoretical evidence is sufficiently strong as to warrant careful experimental studies. Such bound excited electronic states could play important roles in the chemistry of the upper atmosphere and in the chemistry of oxygen discharge systems. One (or more) of these states may be responsible for the short-lived intermediate (‘ozone precursor’) recently observed in oxygen radiolysis.     

铍硼四原子簇合物的量子化学从头算

铍和硼簇合物的实验和理论研究表明,它们有特殊的成键方式,BBe2,B2Be2,B6Be和B4Be等簇合物结构已由晶体粉末法测得^[1]。这类化合物可用于带电粒子的活化分析和低能电子的遏止实验,有一定应用前景^[2],对BnBem簇电子结构和成键性质进行研究,不仅对理解大簇和凝聚相的形成机制及微观结构等有重要意义,而且有一定的应用价值^[3-6],B2Be/B2Be^ 的结构^[7]已有论证。本文选用高精度大基组二次组态相互作用QCISD（T）／6－311G^**方法对BBe3,B2Be2和B3Be的电子结构进行了计算,并在HF／6－31G^*水平上作了频率计算。     

A geometry optimization benchmark using highly correlated wavefunctions (FCI and MRD-CI)

Summary Full configuration interaction (FCI) geometry optimizations have been performed for the X³B₁, a¹A₁, b¹B₁ and c¹A₁ electronic states of CH₂, the X²B₁ and A²A₁ electronic states of NH₂ and the X¹A₁ electronic state of BH₃ using a DZP basis set. The results are compared with those obtained using the MRD-CI method at different levels of theoretical treatment. The agreement between the geometrical parameters optimized with the FCI and MRD-CI methods is very good.     

Assignment on the X,A, B,C, and D states of the C6H5Br+ cation based on high‐level calculations

Geometries, frequencies, and energies of the 1²B₁, 1²A₂, 1²B₂, 2²B₁, 2²B₂, and 1²A₁, of the C₆H₅Br⁺ ion were calculated by using CASSCF and CASPT2 methods in conjunction with an ANO‐RCC basis. The CASPT2//CASSCF adiabatic excitation energies and CASPT2 relative energies for the six states are in good agreement with experiment. The X, A, B, C, and D electronic states of the C₆H₅Br⁺ ion were assigned to be X²B₁, A²A₂, B²B₂, C²B₁, and D²B₂ based on the CASSCF and CASPT2 calculations. The assignment on the D state of the C₆H₅Br⁺ ion is different from the previously published works. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

TiO2基态和激发态的几何结构、激发能和偶极矩

采用二阶微扰理论MP2、密度泛函B3LYP方法和含时密度泛函TD-B3LYP方法分别优化了TiO₂分子的基态¹A₁和六个激发态¹B₂、³B₂、¹B₁、³B₁、¹A₂和³A₂的几何结构. ¹A₁、¹B₂、³B₂、¹B₁和³B₁具有弯曲几何结构, ¹A₂和³A₂具有线性对称结构. 我们发现激发态¹B₂、³B₂、¹B₁和³B₁键偶极矩的数值大小顺序和相应的键角大小顺序完全一致. 另外, 采用完全活化空间自洽场(CASSCF)CASSCF(6,6)、CASSCF(8,8)、多参考组态相互作用(MRCI)和含时密度泛函TD-B3LYP 计算了TiO₂ 分子各激发态的垂直激发能和绝热激发能. 对¹B₂、³B₂和¹B₁三个态, MRCI/CASSCF(6,6) 计算的垂直激发能和绝热激发能与已有的实验值最接近. 对其他三个激发态³B₁、¹A₂和³A₂, 计算的激发能和文献报道的激发能计算值基本一致. 最后, 还计算了TiO₂分子的基态和激发态的偶极矩. 对¹A₁和¹B₂态, 偶极矩的计算值与已有的实验值相吻合. 采用原子偶极矩校正的Hirshfeld 布居方法计算了TiO₂分子在¹A₁、¹B₂、³B₂、¹B₁和³B₁态时各原子的电荷, 发现从基态到激发态偶极矩的变化与电荷从氧原子向钛原子的转移有关. 整个计算中还考察了基函数cc-pVDZ、cc-pVTZ和cc-pVQZ对计算结果的影响.     

The low-lying electronic states of H2CN+: a preliminary ab initio study

Ab initio molecular-orbital calculations have been carried out on the low-lying triplet and singlet electronic states of the H₂CN⁺ cation, at the SCF and Møller-Plesset levels of theory. Both triplet ³A₂ and ³B₂ electronic states have similar energies. The barriers to isomerization to the ³A″ and ³A' electronic states of HCNH⁺ are estimated. It appears that ³A₂ and ³B₂ states are stable towards both isomerization and dissociation. The results of mass spectroscopic experiments involving H₂CN⁺ are discussed.     

The potential energy surfaces of the ground and excited states of carbon dioxide molecule

Potential energy surfaces (PESs) of the ¹A_l(¹Σ _g ⁺ ), ¹B₂ and ³B₂ electronic states of CO₂ have been computed as a function of the two bond distances and the bond angle. The calculations were based on the complete active space self consistent field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) electronic structure models. From our calculations no crossing point between ¹B₂ and ³B₂ states was found, but there is a crossing point located between ¹B₂ and ³A₂ state on the PESs. The energy of the crossing point is lie 0.23 eV above the CO + O (³P), which is in agreement with the value of 0.27 eV on the experiment. This implies that the mechanism of the recombination of an oxygen atom with a carbon monoxide molecule: CO(X ¹Σ⁺, ν) + O(³P)→³CO₂*→¹CO₂*→CO(X ¹Σ⁺, ν = 0) + O(¹ D) may occur through the ³A₂ state crossing the ¹B₂ state. The equilibrium geometries and adiabatic excitation energies of ^1,3B₂, ^1,3A₂ states of CO₂ were reported and discussed in this paper, too.     

Theoretical study of spectroscopic and molecular properties of several low‐lying electronic states of CO molecule

The potential energy curves (PECs) of eight low‐lying electronic states (X¹Σ⁺, a³Π, a′³Σ⁺, d³Δ, e³Σ^?, A¹Π, I¹Σ^?, and D¹Δ) of the carbon monoxide molecule have been studied by an ab initio quantum chemical method. The calculations have been performed using the complete active space self‐consistent field method, which is followed by the valence internally contracted multireference configuration interaction (MRCI) approach in combination with the correlation‐consistent aug‐cc‐pV5Z basis set. The effects on the PECs by the core‐valence correlation and relativistic corrections are included. The way to consider the relativistic corrections is to use the third‐order Douglas–Kroll Hamiltonian approximation at the level of a cc‐pV5Z basis set. Core‐valence correlation corrections are performed using the cc‐pCVQZ basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are corrected for size‐extensivity errors by means of the Davidson modification (MRCI+Q). The spectroscopic parameters (D_e, T_e, R_e, ω_e, ω_ex_e, ω_ey_e, B_e, α_e, and γ_e) of these electronic states are calculated using these PECs. The spectroscopic parameters are compared with those reported in the literature. Using the Breit–Pauli operator, the spin–orbit coupling effect on the spectroscopic parameters is discussed for the a³Π electronic state. With the PECs obtained by the MRCI+Q/aug‐cc‐pV5Z+CV+DK calculations, the complete vibrational states of each electronic state have been determined. The vibrational manifolds have been calculated for each vibrational state of each electronic state. The vibrational level G(ν), inertial rotation constant B_ν, and centrifugal distortion constant D_ν of the first 20 vibrational states when the rotational quantum number J equals zero are reported and compared with the experimental data. Comparison with the measurements demonstrates that the present spectroscopic parameters and molecular constants determined by the MRCI+Q/aug‐cc‐pV5Z+CV+DK calculations are both reliable and accurate. © 2012 Wiley Periodicals, Inc.