Nonadiabatic coupling in the 3 3Pi and 4 3Pi states of NaK

The excited 3 (3)Pi and 4 (3)Pi electronic states of the NaK molecule exhibit an avoided crossing, leading to the anomalous behavior of many features of the rovibrational energy levels belonging to each state. A joint experimental and theoretical investigation of these states has been carried out. Experimental measurements of the vibrational, rotational, and hyperfine structure of numerous levels of the 3 (3)Pi state were recently obtained using the Doppler-free, perturbation-facilitated optical-optical double resonance technique. Additional measurements for the 4 (3)Pi state as well as bound-free emission spectra from selected 3 (3)Pi, 4 (3)Pi, and mixed 3 (3)Pi to approximately 4 (3)Pi rovibrational levels are reported here. A model is also presented for calculating the mixed rovibrational level energies of the coupled 3 (3)Pi-4 (3)Pi system, starting from a 2x2 diabatic electronic Hamiltonian. The 3 (3)Pi and 4 (3)Pi potential curves and the coupling between them are simultaneously adjusted to fit the observed rovibrational levels of both states. The energy levels of the potential curves determined by the fit are in excellent agreement with experiment. The nonadiabatic coupling is sufficiently strong to cause an overall shift of 2-3 cm(-1) for many rovibrational levels as well as somewhat larger shifts for certain pairs of 3 (3)Pi to approximately 4 (3)Pi levels that would otherwise be very close together.     

Computational study of the rovibrational spectrum of (OCS)2

In this paper, we report a new intermolecular potential energy surface and rovibrational transition frequencies and line strengths computed for the OCS dimer. The potential is made by fitting energies obtained from explicitly correlated coupled-cluster calculations and fit using an interpolating moving least squares method. The rovibrational Schroedinger equation is solved with a symmetry-adapted Lanczos algorithm and an uncoupled product basis set. All four intermolecular coordinates are included in the calculation. On the potential energy surface we find, previously unknown, cross-shaped isomers and also polar and non-polar isomers. The associated wavefunctions and energy levels are presented. To identify polar and cross states we use both calculations of line strengths and vibrational parent analysis. Calculated rotational constants differ from their experimental counterparts by less than 0.001 cm(-1).     

The NaK 1 1,3delta states: theoretical and experimental studies of fine and hyperfine structure of rovibrational levels near the dissociation limit

Earlier high-resolution spectroscopic studies of the fine and hyperfine structure of rovibrational levels of the 1 3delta state of NaK have been extended to include high lying rovibrational levels with v < or = 59, of which the highest levels lie within approximately 4 cm(-1) of the dissociation limit. A potential curve is determined using the inverted perturbation approximation method that reproduces these levels to an accuracy of approximately 0.026 cm(-1). For the largest values of v, the outer turning points occur near R approximately 12.7 angstroms, which is sufficiently large to permit the estimation of the C6 coefficient for this state. The fine and hyperfine structure of the 1 3delta rovibrational levels has been fit using the matrix diagonalization method that has been applied to other states of NaK, leading to values of the spin-orbit coupling constant A(v) and the Fermi contact constant b(F). New values determined for v < or = 33 are consistent with values determined by a simpler method and reported earlier. The measured fine and hyperfine structure for v in the range 44 < or = v < or = 49 exhibits anomalous behavior whose origin is believed to be the mixing between the 1 3delta and 1 1delta states. The matrix diagonalization method has been extended to treat this interaction, and the results provide an accurate representation of the complicated patterns that arise. The analysis leads to accurate values for A(v) and b(F) for all values of v < or = 49. For higher v (50 < or = v < or = 59), several rovibrational levels have been assigned, but the pattern of fine and hyperfine structure is difficult to interpret. Some of the observed features may arise from effects not included in the current model.     

Accurate intermolecular ground state potential of the Ne-HCl van der Waals complex

From an accurate ground state intermolecular potential energy surface we evaluate the rovibrational spectrum of the Ne-HCl van der Waals complex. The intermolecular potential is obtained by fitting a considerable number of interaction energies obtained at the coupled cluster singles and doubles including connected triple excitations level and with the augmented correlation consistent polarized valence quintuple zeta basis set extended with a set of 3s3p2d1f1g midbond functions. This basis set is selected after a systematic basis set study carried out at geometries close to those of the three main surface stationary points. The surface is characterized by two linear minima, i.e. Ne-ClH and Ne-HCl, with distances from the Ne atom to the HCl center of mass of 3.398 and 3.833 angstroms, respectively; and binding energies of -65.10 and -66.85 cm(-1), respectively. These results agree well with the experimental data available in contrast to previous theoretical results. The rovibrational spectra calculated for the different isotopic species are also compared to the experiments.     

Nitrous oxide dimer: a new potential energy surface and rovibrational spectrum of the nonpolar isomer

The spectrum of nitrous oxide dimer was investigated by constructing new potential energy surfaces using coupled-cluster theory and solving the rovibrational Schro?dinger equation with a Lanczos algorithm. Two four-dimensional (rigid monomer) global ab initio potential energy surfaces (PESs) were made using an interpolating moving least-squares (IMLS) fitting procedure specialized to describe the interaction of two linear fragments. The first exploratory fit was made from 1646 CCSD(T)/3ZaP energies. Isomeric minima and connecting transition structures were located on the fitted surface, and the energies of those geometries were benchmarked using complete basis set (CBS) extrapolations, counterpoise (CP) corrections, and explicitly correlated (F12b) methods. At the geometries tested, the explicitly correlated F12b method produced energies in close agreement with the estimated CBS limit. A second fit to 1757 data at the CCSD(T)-F12b/VTZ-F12 level was constructed with an estimated fitting error of less than 1.5?cm(-1). The second surface has a global nonpolar O-in minimum, two T-shaped N-in minima, and two polar minima. Barriers between these minima are small and some wave functions have amplitudes in several wells. Low-lying rovibrational wave functions and energy levels up to about 150?cm(-1) were computed on the F12b PES using a discrete variable representation/finite basis representation method. Calculated rotational constants and intermolecular frequencies are in very close agreement with experiment.     

Ab initio potential-energy surface for the reaction Ca+HCl-->CaCl+H

The potential-energy surface of the ground electronic state of CaHCl has been obtained from 6400 ab initio points calculated at the multireference configuration-interaction level and represented by a global analytical fit. The Ca+HCl-->CaCl+H reaction is endothermic by 5100 cm(-1) with a barrier of 4470 cm(-1) at bent geometry, taking the zero energy in the Ca+HCl asymptote. On both sides of this barrier are potential wells at linear geometries, a shallow one due to van der Waals interactions in the entrance channel, and a deep one attributed to the H(-)Ca(++)Cl(-) ionic configuration. The accuracy of the van der Waals well depth, approximately 200 cm(-1), was checked by means of additional calculations at the coupled-cluster singles and doubles with perturbative triples level and it was concluded that previous empirical estimates are unrealistic. Also, the electric dipole function was calculated, analytically fitted in the regions of the two wells, and used to analyze the charge shifts along the reaction path. In the insertion well, 16,800 cm(-1) deep, the electric dipole function confirmed the ionic structure of the HCaCl complex and served to estimate effective atomic charges. Finally, bound rovibrational levels were computed both in the van der Waals well and in the insertion well, and the infrared-absorption spectrum of the insertion complex was simulated in order to facilitate its detection.     

Ab initio ground- and excited-state intermolecular potential energy surfaces for the NO-Ne and NO-Ar van der Waals complexes

The ground- [NO(X(2)Π)] and excited-state [NO(A(2)Σ(+))] intermolecular potential energy surfaces (IPESs) of the NO-Ne and NO-Ar van der Waals complexes are evaluated using the RCCSD(T) spin-restricted coupled cluster method and d-aug-cc-pVQZ basis set extended with a set of 3s3p2d1f1g midbond functions. These bases are selected from the results of a systematic basis-set convergence study carried out for the NO(A(2)Σ(+))-Ar state. We fit the interaction energies to analytic functions and compare the results to those previously available. The NO-Ar (NO-Ne) IPESs are characterized by absolute minima of -120 and -75 cm(-1) (-58 and -5 cm(-1)) at the ground and first excited state, respectively, located close to the T-shaped geometries for the ground states and at linear dispositions in the case of the excited states. The potentials are further used in the evaluation of the rovibrational spectra of the complexes, and the results are compared to those available in the literature.     

A new ab initio interaction energy surface and high-resolution spectra of the H2-CO van der Waals complex

A new four-dimensional intermolecular potential-energy surface for the H(2)-CO complex is presented. The ab initio points have been computed on a five-dimensional grid including the dependence on the H-H separation (the C-O separation was fixed). The surface has then been obtained by averaging over the intramolecular vibration of H(2). The coupled-cluster supermolecular method with single, double, and noniterative triple excitations has been used to calculate the interaction energy. The correlation part of the interaction energy has been obtained from extrapolations based on calculations in a series of basis sets. An analytical fit of the ab initio potential-energy surface has the global minimum of -93.049 cm(-1) at the intermolecular separation of 7.92 bohr for the linear geometry with the C atom pointing toward the H(2) molecule. For the other linear geometry, with the O atom pointing toward H(2), the local minimum of -72.741 cm(-1) has been found for the intermolecular separation of 7.17 bohr. The potential has been used to calculate the rovibrational energy levels of the para-H(2)-CO complex. The results agree very well with those observed by McKellar [A. R. W. McKellar J. Chem. Phys. 108, 1811 (1998)]: the discrepancies are smaller than 0.1 cm(-1). The calculated dissociation energy is equal to 19.527 cm(-1) and significantly smaller than the value of 22 cm(-1) estimated from the experiment. Predictions of rovibrational energy levels for ortho-H(2)-CO have also been done and can serve as a guidance to assign recorded experimental spectra. The interaction second virial coefficient has been calculated and compared with the experimental data.     

Theoretical spectroscopy of the N2HAr+ complex

The six-dimensional potential energy surface of the electronic ground state of N2HAr+ is determined by ab initio computations at the CCSD(T) level of theory. The potential energy surface is used to derive a set of spectroscopic data for N2HAr+ and N2DAr+ using second order perturbation theory. Full six-dimensional (6-D) rotation-vibration computations are also carried out using an analytical representation of the surface for J=0 and 1, in order to deduce the rovibrational spectra of N2HAr+ and its deuterated isotopomer. Our variationally determined anharmonic wavenumbers differ by less than 15 cm(-1) from the most accurate experimental values. Strong anharmonic resonances are found between the rovibrational levels of both cations even at low energies.     

SO2基态势能面和振转能级的理论研究

在键长-键角坐标下,精确求解SO₂的核振动方程,并通过与实验数据比较来优化势能参数,由所得势能面计算得到38个振动能级,与实验值相比,均方根误差为0.93cm^-1.计算了³⁴SO₂的部分振动能级以及³²SO₂的J=6以下的部分振动能级,所得结果均与实验值较为吻合     

Jet-cooled diode laser spectra of CF3Br in the 9.2 microm region and rovibrational analysis of symmetric CF3 stretching mode

Pulsed slit-jet high resolution (up to 0.0009 cm(-1) FWHM) infrared diode laser spectra of CF(3)Br, with natural isotopic abundance, were obtained in the region around 9.2 microm at the rotational temperature of about 50 K. In addition, diode laser spectra at reduced temperature (200 K) were recorded. We present here the rovibrational analysis of the nu(1) fundamental in the range 1075-1090 cm(-1). The fine structure of many P(J) and R(J) clusters has been well resolved for the first time. The assignment of rovibrational transitions has been accomplished up to K = 27, J = 63 for CFBr and K = 33, J = 62 for CFBr. A total of 636 (CFBr) and 880 (CFBr) lines were used in the final fit and a very accurate set of molecular constants, including the quartic centrifugal distortion coefficients for the v(1) = 1 state of both the bromine isotopologues, was obtained. In addition, spectral features belonging to the nu(1) + nu(6)-nu(6) hot band were unambiguously identified and a set of spectroscopic parameters were determined.     

Experimental long range potential of the B 1Pi state in NaRb

The long range potential of the B 1Pi state in NaRb has been investigated by observation of rovibrational levels that it supports, including the high lying ones, with the technique of polarization labeling spectroscopy. This has allowed us to characterize the potential energy curve up to 1.9 cm(-1) from the dissociation limit. The highest observed rovibrational level v=49, J=10 has the outer turning point at R=16.48 A.     

Improving the calculation of rovibrational spectra of five-atom molecules with three identical atoms by using a C3upsilonG6 symmetry-adapted grid: applied to CH3D and CHD3

In this paper we report two improvements on the approach we have used to compute rovibrational levels of methane and apply the new ideas to calculate rovibrational levels of two methane isotopomers CH3D and CHD3. Both improvements make the bend calculation better. The first improvement is a G6-invariant (or C3upsilon-invariant) grid which is designed such that each point on the grid is mapped to another point on the grid by any of the G6 operations. The second improvement is the use of fast Fourier transform (FFT) to compute the bend potential matrix-vector products. The FFT matrix-vector product is about three and ten times faster than the previous sequential summation method for the J=0 and J>0 cases, respectively. The calculated J=1 rovibrational levels of CH3D and CHD3 on the Schwenke and Partridge [Spectrochim. Acta, Part A 57, 887 (2001)] ab initio potential are in good agreement (within 6 cm(-1) for the levels up to 3000 cm(-1)) with the experimental data. The agreement is even better (within 0.1 cm(-1) for the levels up to 6000 cm(-1)) if the associated J=0 energies are subtracted.     

Three-dimensional ab initio potential-energy surface and rovibrational spectra of the H2-Kr complex

We report a three-dimensional ab initio potential-energy surface for the H2-Kr complex calculated using a supermolecular method. The electronic calculations were performed at the coupled-cluster singles and doubles level with noniterative inclusion of connected triples levels with a large basis set including midbond functions and the full counterpoise correction for the basis-set superposition error. The intermolecular potential energy between the H2 molecule and the Kr atom were evaluated at five potential-optimized discrete variable representation (DVR) grid points generated from the potential-energy curve of H2. The potential for other bond lengths of H2 could be deduced using polynomial interpolations. The complex is found to have a linear preferred structure with a rather flat energy barrier. The three-dimensional DVR method and the Lanczos propagation algorithm were employed to calculate the rovibrational states without separating the inter- and intramolecular nuclear motions. In addition, the rovibrational spectra from the H2 fundamental vibrational band were calculated. The calculated shift for the band origin is -1.50 cm-1, which is in good agreement with the experimental value of -1.706 cm-1, and the calculated transition frequencies in Q1(0) and S1(0) bands are within 3% of the observed values.     

A perturbative calculation of the rovibrational energy levels of methane

The rovibrational energy levels of methane are determined from a quartic ab initio potential energy force field where the expansion coordinates are the Morse coordinates for the stretches and extension coordinates for the bends. Energies are calculated using canonical Van Vleck perturbation theory. Results are obtained for both rotation-vibration Hamiltonians expressed as functions of curvilinear and rectilinear normal coordinates. Second, fourth, and sixth order curvilinear results are compared with experimental results, and fourth order results for the rectilinear and curvilinear Hamiltonian are compared to each other. The calculated rovibrational levels are in good agreement with the experimental values for low J levels. The calculated rotational level splittings are in even better agreement with the experiment. In particular, the ground state tetrahedral splittings, which are as small as 10(-4) cm(-1), are well reproduced by our calculations at sixth order.     

Potential energy surface and quantum dynamics study of rovibrational states for HO(3) (X (2)A')

An analytic potential energy surface has been constructed by fitting to about 28 thousand energy points for the electronic ground-state (X (2)A') of HO(3). The energy points are calculated using a hybrid density functional HCTH and a large basis set aug-cc-pVTZ, i.e., a HCTH/aug-cc-pVTZ density functional theory (DFT) method. The DFT calculations show that the trans-HO(3) isomer is the global minimum with a potential well depth of 9.94 kcal mol(-1) with respect to the OH + O(2) asymptote. The equilibrium geometry of the cis-HO(3) conformer is located 1.08 kcal mol(-1) above that of the trans-HO(3) one with an isomerization barrier of 2.41 kcal mol(-1) from trans- to cis-HO(3). By using this surface, a rigorous quantum dynamics (QD) study has been carried out for computing the rovibrational energy levels of HO(3). The calculated results determine a dissociation energy of 6.15 kcal mol(-1), which is in excellent agreement with the experimental value of Lester et al. [J. Phys. Chem. A, 2007, 111, 4727.].     

Very accurate potential energy curve of the He2+ ion

A very accurate ground-state potential energy curve (PEC) of the He(2)(+) molecule is calculated with 1200 explicitly correlated Gaussian functions with shifted centers in the range between 0.9 and 100 a(0). The calculations include the adiabatic corrections determined for the (3)He(4)He(+), (3)He(2)(+), and (4)He(2)(+) isotopologues. The absolute accuracy of the PEC is better than 0.05 cm(-1) and that of the adiabatic corrections is around 0.01 cm(-1). The depths of the PECs augmented with the adiabatic corrections for the three isotopologues are: 19 956.708 cm(-1) for (4)He(2)(+), 19 957.054 cm(-1) for (3)He(4)He(+), and 19 957.401 cm(-1) for (3)He(2)(+). The rovibrational energies are also determined. For (3)He(4)He(+) the computed rovibrational transitions corresponding to the ν = 1-0 band differ from the experiment by less than 0.005 cm(-1). For the rovibrational transitions corresponding to the ν = 23-22 band the difference is around 0.012 cm(-1). Presently, this represents the best agreement between theory and experiment for He(2)(+).     

Ab initio potential energy surface and rovibrational spectrum of Ar-HCCCN

We report an ab initio intermolecular potential energy surface of the Ar-HCCCN complex using a supermolecular method. The calculations were performed using the fourth-order M?ller-Plesset theory with the full counterpoise correction for the basis set superposition error and a large basis set including bond functions. The complex was found to have a planar T-shaped structure minimum and a linear minimum with the Ar atom facing the H atom. The T-shaped minimum is the global minimum with the well depth of 236.81 cm(-1). A potential barrier separating the two minima is located at R=5.57 A and theta=20.39 degrees with the height of 151.59 cm(-1). The two-dimensional discrete variable representation was employed to calculate the rovibrational energy levels for Ar-HCCCN. The rovibrational spectra including intensities for the ground state and the first excited intermolecular vibrational state are also presented. The results show that the spectra are mostly b-type (Delta K(a)=+/-1) transitions with weak a-type (Delta K(a)=0) transitions in structure, which are in good agreement with the recent experimental results [A. Huckauf, W. Jager, P. Botschwina, and R. Oswald, J. Chem. Phys. 119, 7749 (2003)].     

A new ab initio intermolecular potential energy surface and predicted rotational spectra of the Ne-H2S complex

We report a new three-dimensional ab initio intermolecular potential energy surface for the Ne-H(2)S complex with H(2)S monomer fixed at its experimental average structure. Using the supermolecular approach, the intermolecular potential energies were evaluated at CCSD(T) (coupled cluster with single and double and perturbative triple excitations) level with large basis sets including bond functions. The full counterpoise procedure was employed to correct the basis set superposition error. The planar T-shaped global minimum is located at the intermolecular distance of 3.51 ? with a well depth of 71.57 cm(-1). An additional planar local minimum was found to be separated from the global minimum with an energy barrier of 23.11 cm(-1). In addition, two first-order and one second-order saddle points were also located. The combined radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm were employed to evaluate the rovibrational energy levels for eight isotopic species of the Ne-H(2)S complexes. The rotational transition frequencies for the eight isotopomers were also determined for the ground and first vibrational excited states, which are all in very good agreement with the available experimental values.     

Five-dimensional ab initio potential energy surface and predicted infrared spectra of H2-CO2 van der Waals complexes

The authors present a new five-dimensional potential energy surface for H2-CO2 including the Q3 normal mode for the nu3 antisymmetric stretching vibration of the CO2 molecule. The potential energies were calculated using the supermolecular approach with the full counterpoise correction at the CCSD(T) level with an aug-cc-pVTZ basis set supplemented with bond functions. The global minimum is at two equivalent T-shaped coplanar configurations with a well depth of 219.68 cm-1. The rovibrational energy levels for four species of H2-CO2 (paraH2-, orthoH2-, paraD2-, and orthoD2-CO2) were calculated employing the discrete variable representation (DVR) for radial variables and finite basis representation (FBR) for angular variables and the Lanczos algorithm. Our calculations showed that the off-diagonal intra- and intermolecular vibrational coupling could be neglected, and separation of the intramolecular vibration by averaging the total Hamiltonian with the wave function of a specific vibrational state of CO2 should be a good approximation with high accuracy. The calculated band origin shift in the infrared spectra in the nu3 region of CO2 is -0.113 cm-1 for paraH2-CO2 and -0.099 cm-1 for orthoH2-CO2, which agrees well with the observed values of -0.198 and -0.096 cm-1. The calculated rovibrational spectra for H2-CO2 are consistent with the available experimental spectra. For D2-CO2, it is predicted that only a-type transitions occur for paraD2-CO2, while both a-type and b-type transitions are significant for orthoD2-CO2.