Ab initio configuration-interaction investigation of optical transitions in K+He and K+H2

The potassium resonance line (4s-->4p) centered around 770 nm is a major contributor to the optical extinction in the atmospheres of certain classes of brown dwarfs and extrasolar giant planets. The resonance line is significantly broadened by collisions with He and H2, and an accurate calculation of the line profile is needed for astrophysical models of these objects. As a first step, we report an accurate ab initio study of the K+He and K+H2 potential-energy curves correlating to the K 4s and 4p atomic energy levels, together with the dipole moments governing the transitions between these potential-energy curves. The molecular calculations have been carried out using a multireference configuration-interaction method, with the molecular orbitals expanded in a large Gaussian basis set. The transition dipole moments show significant variation with the molecular geometry. Calculations for the K+H2 system have been carried out for a range of H2 orientations and internuclear separations, so that the effect of H2 rotation and vibration may be explicitly included in future calculations of the pressure-broadened line profiles.     

Ab initio interaction and spectral properties of CO+–He

In this contribution, ab initio methods have been used to study the open-shell CO⁺–He van der Waals (vdW) complex in both the ground and the first Π excited electronic state. Calculations were performed at the UCCSD(T) level of theory in the framework of the supermolecule approach using the cc-pVTZ basis set complemented with a set of standard bond functions in the middle of the vdW bond. Calculations predict a most-stable equilibrium conformation with β e=45°, R _e =2.85 Å and D _e =275 cm^?1 for the ground CO⁺(X²Σ)–He(¹S) state and β e=90°, R _e =2.70 Å and D _e =218 cm^?1 for the excited CO ⁺(A²Π)–He(¹S) state. The dipole moment μ and independent components of the field polarizability α of the CO ⁺–He vdW complex have been studied at the calculated equilibrium geometry of these states. The vertical excitation energies from the ground CO⁺(X ²Σ)–He(¹S) to the excited CO⁺(A²Π)–He (¹S) electronic state and corresponding shifts in the fluorescent spectrum with respect to the isolated CO⁺ molecule are also presented     

Ab initio calculations of the rotationally resolved infrared spectrum of KNa 2 +

Summary Ab initio variational calculations were performed on the rotationally resolved infrared spectrum of KNa ₂ ⁺ . A discrete potential energy surface was generated using the configuration interaction ansatz coupled with the frozen core approximation, from which an analytical representation was obtained using a power series expansion employing a Dunham expansion variable. This force field was embedded in an Eckart-Watson rovibrational Hamiltonian, from which eigenfunctions and eigenenergies were calculated. An SCF dipole moment surface was generated and used to calculate absolute line intensities and square dipole matrix elements between the vibrational ground state and the lowest-lying excited states for some of the most intense transitions within the P, Q and R branches.     

Ab initio calculation of the rotational spectrum of methane vibrational ground state

In a previous article we have introduced an alternative perturbation scheme to the traditional one starting from the harmonic oscillator, rigid rotator Hamiltonian, to find approximate solutions of the spectral problem for rotation-vibration molecular Hamiltonians. The convergence of our method for the methane vibrational ground state rotational energy levels was quicker than that of the traditional method, as expected, and our predictions were quantitative. In this second article, we study the convergence of the ab initio calculation of effective dipole moments for methane within the same theoretical frame. The first order of perturbation when applied to the electric dipole moment operator of a spherical top gives the expression used in previous spectroscopic studies. Higher orders of perturbation give corrections corresponding to higher centrifugal distortion contributions and are calculated accurately for the first time. Two potential energy surfaces of the literature have been used for solving the anharmonic vibrational problem by means of the vibrational mean field configuration interaction approach. Two corresponding dipole moment surfaces were calculated in this work at a high level of theory. The predicted intensities agree better with recent experimental values than their empirical fit. This suggests that our ab initio dipole moment surface and effective dipole moment operator are both highly accurate.     

Ab initio simulation of the vibrationally resolved photoelectron spectrum of Si3-

Electron photodetachment spectra provide a wealth of information about the electronic and vibrational level structures of neutral molecules that form stable anions. Experiments carried out for the smallest polyatomic silicon cluster anion (Si3-+hupsilon-->Si3*+e-) show vibrational progressions in six observed electronic bands (X-E) of the neutral species. The authors have performed ab initio calculations using the MRCI+D/aug-cc-pVQZ level for the corresponding electronic states followed by variational calculations of the vibronic levels associated with these adiabatic potential energy surfaces. In contrast to previous approaches, the authors treat the nonadiabatic dynamics on the potential energy surfaces, which allows for a vastly improved reproduction of the experimental level structure and a corrected assignment for band A.     

Ab initio ground state phenylacetylene-argon intermolecular potential energy surface and rovibrational spectrum

We evaluate the phenylacetylene-argon intermolecular potential energy surface by fitting a representative number of ab initio interaction energies to an analytic function. These energies are calculated at a grid of intermolecular geometries, using the CCSD(T) method and the aug-cc-pVDZ basis set extended with a series of 3s3p2d1f1g midbond functions. The potential is characterized by two equivalent global minima where the Ar atom is located above and below the phenylacetylene plane at a distance of 3.5781 A? from the molecular center of mass and at an angle of 9.08° with respect to the axis perpendicular to the phenylacetylene plane and containing the center of mass. The calculated interaction energy is -418.9 cm(-1). To check further the potential, we obtain the rovibrational spectrum of the complex and the results are compared to the available experimental data.     

Ab initio spectrograms of the molecular vibrational spectrum

Theoretical spectrograms of the vibrational spectrum of 3,3-dimethylcyclopropene were constructed and juxtaposed with the experimental Raman and IR spectrograms. The theoretical spectrograms are represented as sets of vertical lines starting from the points corresponding to the values of the vibrational frequencies calculated from the scaled quantum-mechanical (QM) force field obtained at the HF/6-31G*//HF/6-31G* level. Two theoretical Raman spectrograms were constructed. In the first case, the heights of the vertical lines correspond to the QM values of the Raman scattering activities. In the second case they represent the relative differential Raman cross-sections calculated using the QM values of Raman scattering activities. The initial vibrational mode matrix remains virtually unchanged upon scaling of the QM force constant matrix because the dispersion of the scale factor values is low. Therefore, the heights of the theoretical lines for the IR spectrogram represent the QM intensities directly. The theoretical spectrogram based on the relative differential Raman cross-sections was shown to depict the experimental Raman spectrum more adequately. This makes it possible to use the results of the corresponding QM calculations more completely and obtain well-substantiated assignments of the vibrational frequencies.     

Calculated ro-vibrational spectrum of 7Li+3 and 7Li2 6Li+

Calculated ro-vibrational energy levels (J ⩽ 4) and transition intensities are presented for the two most abundant isotopomers of Li⁺₃. The calculations use the recent ab initio potential energy surface of Searles et al. (Spectrochim. Acta 43A, 699 (1987); 44A, 505 (1988); 44A, 985 (1988)). The rotational levels of the ground state and vibrational fundamentals are given in terms of parameterised Hamiltonians due to Watson retaining terms to fourth-order. The small splitting of the degenerate ν₂ mode in the mixed isotopomer leads to strong Coriolis coupling between the ν₂ and ν₃ in ⁷Li₂ ⁶Li⁺.     

Ab initio potential energy surface and spectrum of the B(3Pi) state of the HeI2 complex

The three-dimensional interaction potential for I2(B 3Pi0u+)+He is computed using accurate ab initio methods and a large basis set. Scalar relativistic effects are accounted for by large-core relativistic pseudopotentials for the iodine atoms. Using multireference configuration interaction calculations with subsequent treatment of spin-orbit coupling, it is shown for linear and perpendicular structures of the complex that the interaction potential for I2(B 3Pi0u+)+He is very well approximated by the average of the 3A' and 3A" interaction potentials obtained without spin-orbit coupling. The three-dimensional 3A' and 3A" interaction potentials are computed at the unrestricted open-shell coupled-cluster level of theory using large basis sets. Bound state calculations based on the averaged surface are carried out and binding energies, vibrationally averaged structures, and frequencies are determined. These results are found to be in excellent accord with recent experimental measurements from laser-induced fluorescence and action spectra of HeI2. Furthermore, in combination with a recent X-state potential, the spectral blueshift is obtained and compared with available experimental values.     

Ab initio prediction of the infrared-absorption spectrum of the C2Cl radical

The three lowest (1(2)A', 2(2)A', and 1(2)A") potential-energy surfaces of the C2Cl radical, correlating at linear geometries with 2Sigma+ and 2Pi states, have been studied ab initio using a large basis set and multireference configuration-interaction techniques. The electronic ground state is confirmed to be bent with a very low barrier to linearity, due to the strong nonadiabatic electronic interactions taking place in this system. The rovibronic energy levels of the 12C12C35Cl isotopomer and the absolute absorption intensities at a temperature of 5 K have been calculated, to an upper limit of 2000 cm(-1), using diabatic potential-energy and dipole moment surfaces and a recently developed variational method. The resulting vibronic states arise from a strong mixture of all the three electronic components and their assignments are intrinsically ambiguous.     

Accurate potential energy curve for B2. Ab initio elucidation of the experimentally elusive ground state rotation-vibration spectrum

The electron-deficient diatomic boron molecule has long puzzled scientists. As yet, the complete set of bound vibrational energy levels is far from being known, experimentally as well as theoretically. In the present ab initio study, all rotational-vibrational levels of the X (3)Σ(g)(-) ground state are determined up to the dissociation limit with near-spectroscopic accuracy (<10 cm(-1)). Two complete sets of bound vibrational levels for the (11)B(2) and (11)B-(10)B isotopomers, containing 38 and 37 levels, respectively, are reported. The results are based on a highly accurate potential energy curve, which also includes relativistic effects. The calculated set of all vibrational levels of the (11)B(2) isotopomer is compared with the few results derived from experiment [Bredohl, H.; Dubois, I.; and Nzohabonayo, P. J. Mol. Spectrosc. 1982, 93, 281; Bredohl, H.; Dubois, I.; and Melen, F. J. Mol. Spectrosc. 1987, 121, 128]. Theory agrees with experiment within 4.5 cm(-1) on average for the four vibrational level spacings that are so far known empirically. In addition, the present theoretical analysis suggests, however, that the transitions from higher electronic states to the ground state vibrational levels v = 12-15 deserve to be reanalyzed. Whereas previous experimental investigators considered them to originate from the v' = 0 vibrational level of the upper state (2)(3)Σ(u)(-), the present results make it likely that these transitions originate from a different upper state, namely the v' = 16 or the v' = 17 vibrational level of the (1)(3)Σ(u)(-) state. The ground state dissociation energy D(0) is predicted to be 23164 cm(-1).     

He++N2 Charge Transfer Reaction: An Ab initio Analysis

An ab initio analysis on the involved potential energy surfaces is presented for the investigation of the charge transfer mechanism for the He⁺+N₂ system. At high collision energy, as many as seven low-lying electronic states are observed to be involved in the charge transfer mechanism. Potential energy surfaces for these low-lying electronic states have been computed in the Jacobi scattering coordinates, applying multireference configuration interaction level of theory and aug-cc-pVQZ basis sets. Asymptotes for the ground and various excited states are assigned to mark the entrance (He⁺+N₂) and charge transfer channels (He+N₂⁺). Nonadiabatic coupling matrix elements and quasi-diabatic potential energy surfaces have been computed for all seven states to rationalize the available experimental data on the charge transfer processes and to facilitate dynamics studies.     

Ab initio rovibrational states and infrared spectrum of RbCN

Using an ab initio potential surface the rovibrational states of RbCN are calculated in the atom-(rigid) diatom formalism. From these, infrared transition intensities and vibrationally averaged dipole moments are obtained, using an ab initio dipole surface. The lower vibrational states can be labeled by bend and stretch, for which the fundamental frequencies are 100.4 and 258.0 cm⁻¹. At energies higher than 500 cm⁻¹, many overlapping resonances are found and the vibrational labeling breaks down. The calculated ground-state rotational constants are A = 2.269 cm⁻¹, B = 0.106 cm⁻¹ and C = 0.100 cm⁻¹, with an inertial defect ΔI = 0.687 amu Ų. For the higher vibrational states these parameters are used to study the increasing floppyness of the molecule and its behaviour as an effective linear isocyanide in excited states. At low temperature, the vibrational absorption spectrum only contains the fundamental transitions plus a transition caused by a Fermi resonance between the stretch fundamental and the third bending overtone. At high temperature, the chaotic and quasi-linear states have a marked effect on the absorption spectrum.     

Ab initio study of the electronic spectrum of peroxyacetyl nitrate

A theoretical study of the ground and excited states of peroxyacetyl nitrate (PAN), CH3C(O)OONO2, has been carried out using high level ab initio molecular orbital methods. The ground state geometry and vibrational frequencies are calculated using the coupled-cluster method. The vertical excitation energies for the lowest three excited states are calculated using the complete active space self-consistent field method along with the multireference internally contracted configuration interaction method. These results are compared with vertical excitation energies calculated with the coupled cluster equation of motion method. The calculation provides relevant insight into the origin of PAN absorption in the UV wavelength region from 200 to 300 nm. The nature of the electron transitions for these excited states is discussed.     

Ab initio study of the KrH+ photodissociation

The multireference spin-orbit configuration interaction method is employed to calculate potential energy curves for the ground and low-lying excited states of the KrH(+) cation. For the first time, the spin-orbit interaction is taken into account and electric dipole moments are computed for transitions to the states responsible for the first absorption continuum (A band) of KrH(+). On this basis, the partial and total absorption spectra in this energy range are obtained. It is shown that the A-band absorption is dominated by the parallel A (1)Sigma(+)<--X (1)Sigma(+) transition. In the low-energy part of the band (<83x10(3) cm(-1)) the absorption is mainly caused by the spin-forbidden b (3)Pi(0(+) )<--X (1)Sigma(+) excitation, while perpendicular transitions to the B (1)Pi and b (3)Pi(1) states are significantly weaker. The branching ratio Gamma for the photodissociation products is calculated and it is shown to increase smoothly from 0 in the red tail of the band to 1 at E>or=90x10(3) cm(-1). The latter value corresponds to the exclusive formation of the spin-excited Kr(+)((2)P(12)) ions, which may be used to obtain laser generation on the Kr(+)((2)P(12)-(2)P(32)) transition.     

Ab initio study on the mechanism of reaction HNCO+NH_2

Ab initio UMP2 and UQCISD(T) calculations, with 6-311G** basis sets, were performed for the titled reactions. The results show that the reactions have two product channels: NH2+ HNCO→NH3+NCO (1) and NH2+HNCO-N2H3+CO (2), where reaction (1) is a hydrogen abstraction reaction via an H-bonded complex (HBC), lowering the energy by 32.48 kJ/mol relative to reactants. The calculated QCISD(T)//MP2(full) energy barrier is 29.04 kJ/mol, which is in excellent accordance with the experimental value of 29.09 kJ/mol. In the range of reaction temperature 2300-2700 K, transition theory rate constant for reaction (1) is 1.68 × 1011- 3.29 × 1011 mL · mol-1· s-1, which is close to the experimental one of 5.0 ×1011 mL× mol-1· s-1 or less. However, reaction (2) is a stepwise reaction proceeding via two orientation modes, cis and trans, and the energy barriers for the rate-control step at our best calculations are 92.79 kJ/mol (for cis-mode) and 147.43 kJ/mol (for trans-mode), respectively, which is much higher than     

Ab initio study of the electronic spectrum of dichlorocarbene CCl2

The equilibrium geometries, excitation energies, force constants and vibrational frequencies for seven low-lying electronic states X ¹A₁, ¹B₁, ³B₁, ¹A₂, ³A₂, ¹B₂ and ³B₂ of dichlorocarbene CCl₂ have been calculated at the MRSDCI level with a double-zeta plus polarization basis set. Our calculated equilibrium geometry for the X ¹A₁ state, excitation energy for X ¹A₁ → ¹B₁ and vibrational frequencies for the X ¹A₁ and ¹B₁ states are in good agreement with experimental data. The electronic transition dipole moments, oscillator strengths for the ¹B₁ → X ¹A₁ and ¹B₂ → X ¹A₁ transitions, radiative lifetimes for the ¹B₁ and ¹B₁ states are calculated using MRSDCI wavefunctions, predicting results in reasonable agreement with experiment.     

Ab initio embedded cluster study of the excitation spectrum and Stokes shifts of Bi3+-doped Y2O3

The ab initio embedded cluster method coupled with correlated spin-orbit calculations has been used to interpret the excitation spectrum of a Bi(3+)-doped yttria crystal. Our results indicate that the Bi(3+) impurity can absorb light over a wider energy range in the C(2) site than in the S(6) site. Even if the computed absorption energies seem to be about 0.4 eV too high with respect to the experimental peaks for both sites, it is noteworthy that the embedded cluster model renders 93% of the large crystal redshift, about 6 eV. The determination of the geometry relaxation of the first shell of oxygen neighbors upon electronic excitation shows that the Stokes shift is smaller in the S(6) site than in the C(2) site. Combining all these results confirms the assignment of the violet emission to the S(6) site and that of the green emission to the C(2) site, as proposed by Boulon [J. Phys. (Paris) 32, 333 (1971)]. In addition, the nature of the metastable states which lie below the emitting ones and are responsible for the temperature dependence of the fluorescence lifetimes is discussed.     

富勒烯结构Si60的从头算研究

采用量子化学从头算方法, 系统地研究了Si60-Ih及其各种降低对称性后的扭曲构型的稳定性. 找到了5个低能量低对称性(对称性分别为T, Ci, C1, CS和C2) Si60的稳定结构. 分析计算结果表明, 典型的低能量Si60结构对应着一些硅原子凸出球外和一些硅原子凹进球内, 部分Si原子间的成键呈sp3杂化方式.