Search

We have developed a new program for carrying out improved internally contracted Multi-reference Configuration Interaction Singles and Doubles (i2cMRCISD) calculations. It is designed from the ground up to be used on distributed memory parallel computers. Tests show good scaling properties with the number of cores per node and the number nodes. This program features Gaussian basis sets with ℓ > 6; scalar special relativity via the spin-free method; convergence to C∞v, D∞v, or spherical electronic states; special code to determine Rydberg orbitals; both uncontracted and contracted MRCISD wavefunctions; one and two electron properties, including full spin–orbit matrix elements with the Breit interaction; analytic calculation of Born–Oppenheimer diagonal correction for multi-configuration Hartree–Fock wavefunctions; and analytic calculation of second order Born–Oppenheimer corrections for Hartree–Fock wavefunctions. The program can be obtained from software.nasa.gov. [ABSTRACT FROM AUTHOR]

To facilitate the data analysis of current and future high-resolution space telescope missions, we adopt 'Best Theory + Reliable High-resolution Experiment' (BTRHE) strategy to develop highly accurate infrared line lists for nitrous oxide (N2O). The 'Ames-1' potential energy surface (PES) is a CCSD(T)/aug-cc-pVQZ PES refined using selected HITRAN experimental data, with σrms = 0.02–0.03 cm-1 for five isotopologues. The 'Ames-1' dipole moment surface (DMS) is fitted from CCSD(T)/aug-cc-pV(T,Q,5)Z dipoles extrapolated to one electron basis set limit. Using the Ames-1 PES and DMS, Ames-296K line lists are computed in the full range of 0–15,000 cm-1 for 12 N2O isotopologues, with S296K ≥ 10−31 cm-1/molecule.cm-2. The reliability and consistency of Ames-296K intensity predictions (SAmes) are demonstrated through comparisons with HITRAN (SHITRAN), NOSL-296 (SNOSL), recent observed intensities (Sobs) and Effective Dipole Model (EDM) intensities (SEDM). Agreements and discrepancies are discussed, along with preliminary uncertainty estimate for SAmes. The SAmes provides a good constraint to prevent substantial errors in intensity predictions (e.g. for weak bands and minor isotopologues) and can be further improved. Ames-296K and NOSL-296 may complement each other to provide improved input for future database updates, combining the strengths of EH/EDM and BTRHE approaches. Data available at and https://doi.org/10.48667/9kmk-0334. [ABSTRACT FROM AUTHOR]

In this work, we systematically derive the matrix elements of the nuclear rotation operators for open shell diatomic and polyatomic molecules in a parity adapted Hund's case (a) basis. Our expressions are valid for an arbitrary number of electrons and arbitrary electronic configurations. The common ad hoc sign changes of angular momentum operators are shown to be equivalent to a change in phase of basis functions. We show how to relate this basis to that required for scattering calculations. We also give the expressions for Einstein A coefficients for electric dipole, electric quadrupole, and magnetic dipole transitions. [ABSTRACT FROM AUTHOR]

A purely ab initio potential energy surface (PES) was refined with selected 32S16O2 HITRAN data. Compared to HITRAN, the root-mean-squares error (σRMS) for all J = 0-80 rovibrational energy levels computed on the refined PES (denoted Ames-1) is 0.013 cm-1. Combined with a CCSD(T)/aug-cc-pV(Q+d)Z dipole moment surface (DMS), an infrared (IR) line list (denoted Ames-296K) has been computed at 296 K and covers up to 8000 cm-1. Compared to the HITRAN and CDMS databases, the intensity agreement for most vibrational bands is better than 85%- 90%. Our predictions for 34S16O2 band origins, higher energy 32S16O2 band origins and missing 32S16O2 IR bands have been verified by most recent experiments and available HITRAN data. We conclude that the Ames-1 PES is able to predict 32/34S16O2 band origins below 5500 cm-1 with 0.01-0.03 cm-1 uncertainties, and the Ames-296K line list provides continuous, reliable and accurate IR simulations. The Ka-dependence of both line position and line intensity errors is discussed. The line list will greatly facilitate SO2 IR spectral experimental analysis, as well as elimination of SO2 lines in high-resolution astronomical observations. [ABSTRACT FROM AUTHOR]

An isotopic-independent, highly accurate potential energy surface (PES) has been determined for CO2 by refining a purely ab initio PES with selected, purely experimentally determined rovibrational energy levels. The purely ab initio PES is denoted Ames-0, while the refined PES is denoted Ames-1. Detailed tests are performed to demonstrate the spectroscopic accuracy of the Ames-1 PES. It is shown that Ames-1 yields σrms (root-mean-squares error) = 0.0156 cm-1 for 6873 J = 0-117 12C16O2 experimental energy levels, even though less than 500 12C16O2 energy levels were included in the refinement procedure. It is also demonstrated that, without any additional refinement, Ames-1 yields very good agreement for isotopologues. Specifically, for the 12C16O2 and 13C16O2 isotopologues, spectroscopic constants Gv computed from Ames-1 are within ±0.01 and 0.02 cm-1 of reliable experimentally derived values, while for the 16O12C18O, 16O12C17O, 16O13C18O, 16O13C17O, 12C18O2, 17O12C18O, 12C17O2, 13C18O2, 13C17O2, 17O13C18O, and 14C16O2 isotopologues, the differences are between ±0.10 and 0.15 cm-1. To our knowledge, this is the first time a polyatomic PES has been refined using such high J values, and this has led to new challenges in the refinement procedure. An initial high quality, purely ab initio dipole moment surface (DMS) is constructed and used to generate a 296 K line list. For most bands, experimental IR intensities are well reproduced for 12C16O2 using Ames-1 and the DMS. For more than 80% of the bands, the experimental intensities are reproduced with σrms(ΔI) < 20% or σrms(ΔI/δobs) < 5. A few exceptions are analyzed and discussed. Directions for future improvements are discussed, though it is concluded that the current Ames-1 and the DMS should be useful in analyzing and assigning high-resolution laboratory or astronomical spectra. [ABSTRACT FROM AUTHOR]

Several aspects of ammonia rovibrational spectra have been investigated using the new HSL-2 potential energy surface that includes an approximate correction for nonadiabatic effects. The unprecedented accuracy of rovibrational energy levels and transition energies computed using HSL-2 was demonstrated in Part I of this study. For 14NH3, new assignments for a few ν3 + ν4 band transitions and energy levels are suggested, and discrepancies between computed and HITRAN energy levels in the 2ν4 band are analyzed (2ν4 is the most difficult band below 5000 cm-1). New assignments are suggested for existing or missing 2ν4 levels. Several new vibrational bands are identified from existing, unassigned HITRAN data, including 2ν2 + ν4, (ν3 + ν4) -A′/A″, ν1 + 2ν2, and 2ν2 + 2ν4. The strong mixing between the 2ν4 and 2ν2 + ν4 bands is carefully examined and found to be the source of the difficulties in the experimental modeling of 2ν4. Discussion is presented for preliminary J = 10 results, where the overall root-mean-square error is estimated to be less than 0.039 cm-1. The analysis of the 4ν2 band demonstrates both the reliability and the accuracy of predictions from HSL-2. The full list of computed J = 0 band origins (with assignments) and the inversion splittings up to 7000-8000 cm-1 above the zero-point energy are presented. J = 0-2 levels are reported for those bands below 5100 cm-1 that are missing from the HITRAN database. For 15NH3, excellent agreement is found for the available ν2 and ν3 + ν4(E) transition energies, but significant deficiencies are shown for HITRAN levels and several corrections are suggested. The 15N isotopic effects are presented for the J = 0-6 levels of 13 HITRAN bands. For 14ND3, we reproduce the pure rotational inversion spectra line frequencies with an accuracy similar to that for 14NH3. However, it is not possible to reproduce simultaneously all four pairs of inversion-split vibrational fundamentals to better than 0.05 cm-1 uncertainty. It is suggested that a reanalysis of some suspicious 14ND3 fundamental bands is required. The analyses presented here and in Part I show that rovibrational energy levels and transition frequencies computed with HSL-2 (with nonadiabatic corrections) remain highly accurate well beyond the experimental data used in the refinement procedure. Calculations using HSL-2 are capable of revealing many deficiencies in experimental analyses of ammonia spectra and provide reliable predictions with similar accuracy. It is expected that the results of this study will be useful in the future interpretation of high-resolution spectra from laboratory experiments or from astronomical observations. The present work represents a very significant advance in the state of our knowledge of the spectroscopy of ammonia and its isotopologues. [ABSTRACT FROM AUTHOR]

In this work, we build upon our previous work on the theoretical spectroscopy of ammonia, NH3. Compared to our 2008 study, we include more physics in our rovibrational calculations and more experimental data in the refinement procedure, and these enable us to produce a potential energy surface (PES) of unprecedented accuracy. We call this the HSL-2 PES. The additional physics we include is a second-order correction for the breakdown of the Born-Oppenheimer approximation, and we find it to be critical for improved results. By including experimental data for higher rotational levels in the refinement procedure, we were able to greatly reduce our systematic errors for the rotational dependence of our predictions. These additions together lead to a significantly improved total angular momentum (J) dependence in our computed rovibrational energies. The root-mean-square error between our predictions using the HSL-2 PES and the reliable energy levels from the HITRAN database for J = 0-6 and J = 7/8 for 14NH3 is only 0.015 cm-1 and 0.020/0.023 cm-1, respectively. The root-mean-square errors for the characteristic inversion splittings are approximately 1/3 smaller than those for energy levels. The root-mean-square error for the 6002 J = 0-8 transition energies is 0.020 cm-1. Overall, for J = 0-8, the spectroscopic data computed with HSL-2 is roughly an order of magnitude more accurate relative to our previous best ammonia PES (denoted HSL-1). These impressive numbers are eclipsed only by the root-mean-square error between our predictions for purely rotational transition energies of 15NH3 and the highly accurate Cologne database (CDMS): 0.00034 cm-1 (10 MHz), in other words, 2 orders of magnitude smaller. In addition, we identify a deficiency in the 15NH3 energy levels determined from a model of the experimental data . [ABSTRACT FROM AUTHOR]

A global potential energy surface (PES) that includes short and long range terms has been determined for the NH3 molecule. The singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations and the internally contracted averaged coupled-pair functional electronic structure methods have been used in conjunction with very large correlation-consistent basis sets, including diffuse functions. Extrapolation to the one-particle basis set limit was performed and core correlation and scalar relativistic contributions were included directly, while the diagonal Born–Oppenheimer correction was added. Our best purely ab initio PES, denoted “mixed,” is constructed from two PESs which differ in whether the ic-ACPF higher-order correlation correction was added or not. Rovibrational transition energies computed from the mixed PES agree well with experiment and the best previous theoretical studies, but most importantly the quality does not deteriorate even up to 10 300 cm-1 above the zero-point energy (ZPE). The mixed PES was improved further by empirical refinement using the most reliable J=0–2 rovibrational transitions in the HITRAN 2004 database. Agreement between high-resolution experiment and rovibrational transition energies computed from our refined PES for J=0–6 is excellent. Indeed, the root mean square (rms) error for 13 HITRAN 2004 bands for J=0–2 is 0.023 cm-1 and that for each band is always ≤=0.06 cm-1. For J=3–5 the rms error is always ≤=0.15 cm-1. This agreement means that transition energies computed with our refined PES should be useful in the assignment of new high-resolution NH3 spectra and in correcting mistakes in previous assignments. Ideas for further improvements to our refined PES and for extension to other isotopolog are discussed. [ABSTRACT FROM AUTHOR]

We present a detailed ab initio study of the effect that the Born–Oppenheimer diagonal correction (BODC) has on the saddle-point properties of the H3 system and its isotopomers. Benchmark values are presented that are estimated to be within 0.1 cm-1 of the complete configuration-interaction limit. We consider the basis set and correlation treatment requirements for accurate BODC calculations, and both are observed to be more favorable than for the Born–Oppenheimer energies. The BODC raises the H+H2 barrier height by 0.1532 kcal/mol and slightly narrows the barrier—with the imaginary frequency increasing by ∼2%. [ABSTRACT FROM AUTHOR]

We consider the extrapolation of the one-electron basis to the basis set limit in the context of coupled cluster calculations. We produce extrapolation coefficients that produce much more accurate results than previous extrapolation forms. These are determined by fitting to accurate benchmark results. For coupled cluster singles doubles energies, we take our benchmark results from the work of Klopper that explicitly includes the interelectronic distance. For the perturbative triples energies, our benchmark results are obtained from large even-tempered basis set calculations. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We illustrate how one can easily derive kinetic energy operators for polyatomic molecules using polyspherical coordinates with very general choices for z-axis embeddings and angles used to specify relative orientations of internal vectors. Computer algebra is not required. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We compute isotope-independent first- and second-order corrections to the Born-Oppenheimer approximation for water and use them to predict isotopic shifts. For the diagonal correction, we use internally contracted multireference configuration interaction wave functions and derivatives with respect to mass-dependent internal coordinates to generate the mass-independent correction functions. For the nonadiabatic correction, we use a self-consistent field wave function for the ground electronic state and single excitation configuration interaction wave functions for the excited states and a generalization of the Handy, Yamaguchi, and Schaefer method to obtain mass-independent correction functions. We find that including the nonadiabatic correction gives significantly improved results compared to just including the diagonal correction when the BornOppenheimer potential energy surface is optimized for H[sub 2] [sup 16]O. The agreement with experimental results for deuterium- and tritium-containing isotopes is nearly as good as our best empirical correction, however, the present correction is expected to be more reliable for higher, uncharacterized, levels. [ABSTRACT FROM AUTHOR]

For line intensity: • Systematic investigation on the isotopologue consistency of CO 2 and SO 2 IR/MW intensity. • Interesting TDM/Einstein-A patterns found for CO 2 vibrational polyads, SO 2 and CO 2 MW, fundamentals, hotbands, etc. For line positions: • First 2nd-round implementation of BTRHE strategy to minimize the isotope/mass dependent residuals. • –– Two orders of magnitude improvement for SO 2 MW prediction accuracy: 0.01-0.02 MHz (A/B/C), 0-5 MHz (J/Ka<20 lines). • This approach starts from and targets at higher accuracy, can easily extend to other bands or molecules. The semi-empirical molecular rovibrational IR line lists, such as ExoMol, TheoReTs, and Ames, combine the experimental accuracy and theoretical power to reach better than 0.1 cm−1 accuracy for line positions and better than 80–90% agreement for line intensities. The quality of these existing semi-empirical IR lists allows further improvements of intensity and line positions for those unobserved minor isotopologues. This paper presents our new BTRHE (Best Theory + Reliable High-resolution Experiment) strategy implementation. For line intensity, the isotopologue consistency and the patterns of mass dependence in the Ames-296 K SO 2 and CO 2 IR lists are quantitatively presented along the mass-inverse coordinates. The consistency and patterns are better than those in existing experimental data. The methodology proposed here can be used to identify inconsistencies, outliers, and mistakes in intensities, and help improve Effective Dipole Model (EDM) and molecular IR databases. We call for an experimental study on the 50006 and 60007 bands of CO 2 628. For line position predictions, a simple approach combining the variational IR line lists with Effective Hamiltonian (EH) model may refine the effective rotational constants A 0 /B 0 /C 0 and quartic centrifugal distortion constants of minor isotopologues. The prediction accuracy may be improved by two orders of magnitude, i.e. reaching 0–5 MHz prediction accuracy in the range of J < 20–30, K a < 10–20, and 0.01–0.02 MHz accuracy for A 0 /B 0 /C 0. Several important factors have been systematically investigated and discussed, e.g. convergence, uncertainties, higher order terms, fixing EH parameters, mass coordinates, etc. A microwave (MW) line set consisting of 644,636 strong transitions (at 296K) for all 30 isotopologues and corresponding refined EH(Ames) parameters are reported in the supplementary material. This approach may be easily extended to rovibrational bands, hot bands, and other molecular systems. [ABSTRACT FROM AUTHOR]

Highlights • First quantitative validation for the dipole moment surface adopted in Ames-296K SO 2 IR intensity calculations, which integrates variational line lists and effective Hamiltonian and effective dipole model approaches. • Systematic check on the reliability and convergence of vibrational dependence of effective dipole of 13 SO 2 vibrational states. Partition Sum convergence check at 296 K along J and E'. • Isotopologue consistency is superior relative to regular effective dipole models. • Combine the high quality Ames IR intensity with the reliable high-resolution experimental line positions or rovibrational energy levels. • Provide three sets of IR lists for 5 SO 2 isotopologues, the best available alternative for SO 2 high-resolution IR database updates Abstract The quality of Ames-296K SO 2 Infrared (IR) line list intensities is first validated by quantitative exploration of several dipole moment surfaces (DMSs) and partition sum convergence. The DMSs are computed with several of Dunning's correlation-consistent basis sets and their vibrational dependence are compared to the empirical model derived from Stark effect experiments reported by D. Patel, D. Margolese, and T.R. Dyke [ J. Chem. Phys. 70 , 2740 (1979)]. The effective dipole deviations from the DMS adopted in the Ames IR lists is 0.2–0.4% for vibrational states up to 3ν 3. The vibrational dependence of the dipole moment is also in good agreement, except for n ν 1. Partition sum convergence at 296 K is confirmed by new calculations with rotational quantum number J up to 150 and upper state E' up to 8000 cm−1. The isotopologue consistency of the Ames IR line lists is superior relative to the regular Effective Hamiltonian (EH) models and Effective Dipole Moment (EDM) models. The ν 1 + ν 2 and ν 2 + ν 3 intensity consistency check reveals the recently reported experimental intensities need significant improvement or re-analysis. After the accuracy, convergence, and isotopologue consistency have been confirmed, the theoretical Ames-296K intensities are combined with the experimental line positions or EH models that experimental spectroscopists published after 2009. Three high-resolution IR line sets are reported for the 32/33/34S16O 2 , 32S18O 2 and 16O32S18O isotopologues: (1) the "New Lines Sets" include experimentally measured line positions; (2) the "Expanded Line Sets" include possible transitions among new rovibrational levels assigned in experiments and ground state (GS) levels predicted by reliable EH models; (3) the "Ames + MARVEL Sets" include possible transitions among all those levels reported in a recent MARVEL analysis. [Tóbiás et al , JQSRT 208 , 152 (2018)]. Compared to the limited data in High-resolution TRANsmission molecular absorption database (HITRAN), these line sets have significantly improved the data coverage up to 4000 cm−1. Some missing bands can be traced to the unpublished experimental data. The isotopologue consistency of these line sets will help identify the uncertainties and defects in the experimental EH and EDM models. These line sets are good candidates for the next HITRAN update, if line shape parameters are available. The line sets can be downloaded from supplementary files or from the Ames Molecular Spectroscopic Database at http://huang.seti.org. [ABSTRACT FROM AUTHOR]

Focuses on the determination of a high quality ab initio potential energy surface (PES) and dipole moment function for water. Empirical adjustment of the PES to improve the agreement between the computed line positions; Small changes in the PES including an estaimate of core electron correlation which greatly improves the agreement with the experiment.

The quartic force field of cyclic C4 has been computed using basis sets of spdf quality and augmented coupled cluster methods. The effect of core correlation and further basis set extension has been investigated. Vibrational energy levels have been obtained using perturbation theory and two different variational approaches. A severe Fermi resonance exists between the most intense vibration, ν6, and ν3+ν5 through an exceptionally large k356=-258.2 cm-1; a large k1356 =-54.8 cm-1 causes significant higher-order anharmonicity, including a shift in ν6 of +9 cm-1. C4 appears to be an excellent test case for methods for solving the vibrational Schrödinger equation, since perturbation theory breaks down even when the above resonances are accounted for. Our best estimate for ν6, 1320±10 cm-1, may suggest its assignment to a feature detected at 1284 cm-1 in argon and 1302 cm-1 in krypton matrix, but this would imply an unusually large matrix red shift in argon. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper describes the use of an exact fast Fourier transform method to prepare specified vibrational–rotational states of triatomic molecules. The method determines the Fourier coefficients needed to describe the coordinates and momenta of a vibrating–rotating triatomic molecule. Once the Fourier coefficients of a particular state are determined, it is possible to easily generate as many random sets of initial Cartesian coordinates and momenta as desired. All the members of each set will correspond to the particular vibrational–rotational state selected. For example, in the case of the ground vibrational state of a nonrotating water molecule, the calculated actions of 100 sets of initial conditions produced actions within 0.001h of the specified quantization values and energies within 5 cm-1 of the semiclassical eigenvalue. The numerical procedure is straightforward for states in which all the fundamental frequencies are independent. However, for states for which the fundamental frequencies become commensurate (resonance states), there are additional complications. In these cases it is necessary to determine a new set of ‘‘fundamental’’ frequencies and to modify the quantization conditions. Once these adjustments are made, good results are obtained for resonance states. The major problems are in labeling the large number of Fourier coefficients and the presence of regions of chaotic motion. Results are presented for the vibrational states of H2O and HCN and the rovibrational states of H2O. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Le Roy–Hutson intermolecular potential for H2–Ar, which was derived from analysis of spectral data for the van der Waals complex and also shown to predict gas kinetic transport cross sections in good accord with experimental values, was subsequently found to provide rather poor predictions (small by nearly a factor of 2) for vibrational Q-branch Raman line shift cross sections. More recently, an ab initio intermolecular potential for this system has been calculated and scaled to agree with the earlier one in the region of the minimum where the latter is expected to be accurate [D. W. Schwenke et al., J. Chem. Phys. 98, 4738 (1993)]. We show here that this potential predicts line shifts in much better accord with experiment (about 10% large), but its predictions for linewidths are significantly less satisfactory. [ABSTRACT FROM AUTHOR]

Potential energy curves for HeH, NeH, and ArH are determined using large basis sets and high levels of theory. The computed potential energy curves are in excellent agreement with the experimentally determined model potentials for HeH and NeH, but differ with the model potentials deduced for ArH by Tang and Toennies. We derive a potential based on the ab initio calculations and our best estimate of the well depth; using this potential we are able to reproduce the results of the orbiting resonance experiments of Toennies and co-workers. [ABSTRACT FROM AUTHOR]

We describe an analytic representation of the ArH2 potential energy surface which well reproduces the results of extensive ab initio electronic structure calculations. We also give an empirical modification of the function designed to improve agreement with experimental estimates of the van der Waals minimum. The analytic representation smoothly interpolates between the H+H and strong bonding H2 limits. In the fitting process, an accurate reproduction of regions of the potential expected to be important for high temperature (∼3000 K) collision processes is emphasized. Overall, the analytic representation well reproduces the anisotropy and H2 bond length dependence of the input data. [ABSTRACT FROM AUTHOR]

We consider the optimization of the wave functions for coupled vibrations represented by linear combinations of products of functions depending only on a single vibrational coordinate. The functions themselves are optimized as well as the configuration list. For the H2O molecule, highly accurate results are obtained for the lowest 15 levels using significantly shorter expansions than would otherwise be possible. [ABSTRACT FROM AUTHOR]

We have performed extensive ab initio calculations on the ground state potential energy surface of H2+H2O, using a large contracted Gaussian basis set and a high level of correlation treatment. An analytical representation of the potential energy surface was then obtained which reproduces the calculated energies with an overall root-mean-square error of only 0.64 mEh. The analytic representation explicitly includes all nine internal degrees of freedom and is also well behaved as the H2 dissociates; it thus can be used to study collision-induced dissociation or recombination of H2. The strategy used to minimize the number of energy calculations is discussed as well as other advantages of the present method for determining the analytical representation. [ABSTRACT FROM AUTHOR]

This paper presents the results of a detailed study of dissociation and recombination of H2 over the temperature range 1000 to 5000 K. The chemical processes are modeled by solving the master equation for the concentrations of the full set of rovibration states. All of the state-to-state energy transfer rate coefficients required by the master equation are evaluated by the quasiclassical trajectory method using a potential energy surface which is a fit to ab initio electronic structure calculations. The analysis of the results of the master equation to obtain the phenomenological rate coefficients was carried out using several techniques. This is required because there is a mixture of third bodies and their concentrations change as the reaction proceeds. The methods based on one-way fluxes are not reliable, while the methods based upon an eigenvalue solution of the linearized master equation are in reasonable agreement with our preferred method, which is based on fitting the concentrations from a two component master equation. The results from various methods of determining the phenomenological rate coefficient for H2 as a third body are in good agreement with experimental estimates. The recombination rate coefficients are most sensitive to the collision induced dissociation rates of initial states with moderate values of vibrational and rotational quantum numbers. A comparison of the orbiting resonance theory to the accurate results shows that this simple model is not valid under the conditions of the present study. [ABSTRACT FROM AUTHOR]

We present two new analytic potential-energy surfaces suitable for studying the competition between the abstraction reaction H+DCl→HD+Cl and the exchange reaction H+DCl→HCl+D. In the abstraction channel the surfaces are only slightly different from the Stern–Persky–Klein GSW surface, but the exchange barrier on both surfaces is raised by inclusion of a three-center term fitted to ab initio extended-basis-set multireference configuration interaction calculations with scaled external correlation. The two surfaces differ significantly only for the steepness of H–Cl–H bend potential. The exchange and abstraction saddle points are characterized by harmonic analysis for H2Cl, HDCl, and D2Cl, and we also compute vibrationally adiabatic barrier heights including anharmonicity. We also report thermal rate constants and activation energies for both reactions mentioned above. [ABSTRACT FROM AUTHOR]

We construct a new global potential energy hypersurface for H2+H2 and perform quasiclassical trajectory calculations using the resonance complex theory and energy transfer mechanism to estimate the rate of three-body recombination over the temperature range 100–5000 K. The new potential is a faithful representation of ab initio electronic structure calculations, is unchanged under the operation of exchanging H atoms, and reproduces the accurate H3 potential as one H atom is pulled away. Included in the fitting procedure are geometries expected to be important when one H2 is near or above the dissociation limit. The dynamics calculations explicitly include the motion of all four atoms and are performed efficiently using a vectorized variable-stepsize integrator. The predicted rate constants are approximately a factor of 2 smaller than experimental estimates over a broad temperature range. [ABSTRACT FROM AUTHOR]

We present new ab initio calculations of the HF–HF interaction potential for the case where both molecules are simultaneously displaced from their equilibrium internuclear distance. These and previous ab initio calculations are then fit to a new analytic representation which is designed to be efficient to evaluate and to provide an especially faithful account of the forces along the vibrational coordinates. We use the new potential for two sets of quantal scattering calculations for collisions in three dimensions with total angular momentum zero. First we test that the angular harmonic representation of the anisotropy is adequate by comparing quantal rigid rotator calculations to those carried out for potentials involving higher angular harmonics and for which the expansion in angular harmonics is systematically increased to convergence. Then we carry out large-scale quantal calculations of vibration–vibration energy transfer including the coupling of both sets of vibrational and rotational coordinates. These calculations indicate that significant rotational energy transfer accompanies the vibration-to-vibration energy transfer process. [ABSTRACT FROM AUTHOR]

We present three methods for obtaining resonance energies and total and partial widths from scattering matrix elements as a function of energy in the vicinity of an isolated resonance. The methods are applied to 14 resonances in collinear reactive scattering systems, and they yield stable and reasonably consistent results. In 12 of the cases the partial widths sum to significantly less (23%–76%) than the total width even though the eigenphase sum fits the generalized Breit–Wigner form very well. The quantal partial widths are used to compute branching ratios for comparison to approximate results. [ABSTRACT FROM AUTHOR]

We report rigid-rotator close coupling calculations and quasiclassical trajectory calculations for HF–HF collisions with total angular momentum zero. The results are compared to test the trajectory method. [ABSTRACT FROM AUTHOR]

We present a new methodology for solving the three-dimensional reactive scattering problem. It is based upon a convenient choice of coordinates with the arrangement channels coupled either by the Fock method or by the wave function component form of arrangement-channel quantum mechanics. The resulting coupled ordinary differential equations are cast in a form suitable for use with standard nonreactive scattering codes. The method involves no matching of solutions from different arrangements and no numerical derivatives, interpolations of multidimensional functions, or nonlocal kernels. [ABSTRACT FROM AUTHOR]

We report large-basis-set CASSCF/MR-CISD/SEC (complete active space self-consistent-field orbitals used for multireference configuration interaction with all single and double excitations and scaled external correlation) and MP4 (Mo\ller–Plesset fourth order perturbation theory) calculations of the FH2 potential energy surface for collinear and bent geometries in the vicinity of the F---H--H saddle point. These calculations indicate that higher order correlation effects become much more important as the generalized transition states are bent, and that the unrestricted saddle point for this reaction is noncollinear. This means that the sterically allowed cone of reactive configurations is much broader than either previously available ab initio calculations or the present lower-order ones would predict. [ABSTRACT FROM AUTHOR]

We have applied the vibrational-close-coupling, rotational-infinite-order-sudden (VCC-RIOS) and vibrational-rotational close coupling (CC) methods to collisions between He and I2(X 1Σ+g) to study the threshold dependence of vibrational deexcitation cross sections and rates on energy and temperature, respectively, for the v=1→0 and v=16→15 processes, where v is the vibrational quantum number. The long-range attraction causes the cross sections to be decreasing functions of energy below about 10 cm-1 and causes the thermally averaged deexcitation rate constants to have a minimum at about 1 K, but the Wigner threshold law holds only within 10-3 cm-1 of threshold and the Wigner singularity causes the rate constant to assume a nonzero temperature-independent value only below about 10-4 K. We also observe low-energy resonances in the CC calculations and these lead to additional structure in the rate constants but do not change the important conclusions. [ABSTRACT FROM AUTHOR]

We have calculated the interaction energy of two HF molecules at the single-configuration Hartree–Fock level using 34 different basis sets in an effort to assess the reliability and usefulness of the counterpoise correction to account for basis set incompleteness. We find large counterpoise corrections for all configurations studied, and we show that using a large enough basis set so that the counterpoise correction is small does not guarantee accurate results. Furthermore even for smaller basis sets the inclusion of counterpoise corrections does not systematically improve the accuracy of the calculations. [ABSTRACT FROM AUTHOR]

A new 626-based Ames-2 PES refinement and Ames-2016 line lists for 13 CO 2 isotopologues are reported. A consistent σ RMS = ±0.02 cm −1 is established for hundreds of isotopologue band origins using the Ames-2 PES. Ames-2016 line lists are computed at 296 K, 1000 K and 4000 K using the Ames-2 PES and the same DMS-N2 dipole surface used previously, with J up to 150, E ′ up to 24,000 cm −1 or 18,000 cm −1 and appropriate intensity cutoffs. The lists are compared to the CDSD-296, CDSD-4000 databases, UCL line lists, and a few recent highly accurate CO 2 intensity measurements. Both agreements and discrepancies are discussed. Compared to the old Ames CO 2 lists, the Ames-2016 line lists have line position deviations reduced by 50% or more, which consequently leads to more reliable intensities. The line shape parameters in the Ames-2016 line lists are predicted using the newly assigned conventional vibrational polyad quantum numbers for rovibrational levels below 12,000 cm −1 so the quality of the line shape parameters is similar to that of CDSD or HITRAN. This study further proves that a semi-empirically refined PES (Ames-1 and Ames-2) coupled with a high quality ab initio DMS (DMS-N2 and UCL) may generate IR predictions with consistent accuracy and is thus helpful in the analysis of laboratory spectra and simulations of various isotopologues. The Ames-2016 lists based on DMS-N2 have reached the ∼1% intensity prediction accuracy level for the recent 626 30013-00001 and 20013-00001 bands, but further quantification and improvements require sub-percent or sub-half-percent accurate experimental intensities. The inter-isotopologue consistency of the intensity prediction accuracies should have reached better than 1–3% for regular bands not affected by resonances. Since the Effective Dipole Models (EDM) in CDSD and HITRAN have 1–20% or even larger uncertainties, we show that the Ames lists can provide better alternative IR data for many hard-to-determine isotopologue bands. Comparison at 4000 K suggests that the Ames-4000 K 12 C 16 O 2 line list is reliable and consistent within the current cutoffs of J  ≤ 150 and E ′ ≤ 24,000 cm −1 , but intensity contributions involving higher energy levels should not be omitted and future computations need to be converged up to at least 32,000 cm −1 or higher. The remaining issues are discussed regarding the source of energy level discrepancies, intensity underestimations by ∼50% for some weak bands, etc. and also future work. [ABSTRACT FROM AUTHOR]

The low-lying singlet and triplet states of MgO have been studied using a SA-CASCF/IC-MRCI approach using the aug-cc-pV5Z basis set. The spectroscopic constants ( r e , ω e , and T e ) are in good agreement with the available experimental data. The computed lifetime for the B state is in excellent agreement with two of the three experimental results. The d state lifetime is in good agreement with experiment, while the computed D state lifetime is about twice as long as experiment. [ABSTRACT FROM AUTHOR]

By comparing to the most recent experimental data and spectra of the SO 2 628 ν 1 /ν 3 bands (see Ulenikov et al., JQSRT 168 (2016) 29–39), this study illustrates the reliability and accuracy of the Ames-296K SO 2 line list, which is accurate enough to facilitate such high-resolution spectroscopic analysis. The SO 2 628 IR line list is computed on a recently improved potential energy surface (PES) refinement, denoted Ames-Pre2, and the published purely ab initio CCSD(T)/aug-cc-pV(Q+d)Z dipole moment surface. Progress has been made in both energy level convergence and rovibrational quantum number assignments agreeing with laboratory analysis models. The accuracy of the computed 628 energy levels and line list is similar to what has been achieved and reported for SO 2 626 and 646, i.e. 0.01–0.03 cm −1 for bands up to 5500 cm −1 . During the comparison, we found some discrepancies in addition to overall good agreements. The three-IR-list based feature-by-feature analysis in a 0.25 cm −1 spectral window clearly demonstrates the power of the current Ames line lists with new assignments, correction of some errors, and intensity contributions from varied sources including other isotopologues. We are inclined to attribute part of detected discrepancies to an incomplete experimental analysis and missing intensity in the model. With complete line position, intensity, and rovibrational quantum numbers determined at 296 K, spectroscopic analysis is significantly facilitated especially for a spectral range exhibiting such an unusually high density of lines. The computed 628 rovibrational levels and line list are accurate enough to provide alternatives for the missing bands or suspicious assignments, as well as helpful to identify these isotopologues in various celestial environments. The next step will be to revisit the SO 2 828 and 646 spectral analyses. [ABSTRACT FROM AUTHOR]

We report converged quantum mechanical reaction probabilities for O+H2(v=0,1)→OH+H for zero total angular momentum as obtained by an L2 expansion of the reactive amplitude density. These provide a benchmark for testing approximate dynamical theories, and this is illustrated by comparisons to centrifugal sudden distorted wave and least-action calculations and vibrationally adiabatic threshold energies. [ABSTRACT FROM AUTHOR]

Using the latest published, empirically refined potential energy surface (PES) Ames-1 and purely ab initio CCSD(T)/aug-cc-pV(Q+d)Z dipole moment surface (DMS), we have computed infrared line lists for five symmetric isotopologues of sulfur dioxide: 32 S 16 O 2 (626), 33 S 16 O 2 (636), 34 S 16 O 2 (646), 36 S 16 O 2 (666), and 32 S 18 O 2 (828). The line lists are based on J = 0–80 rovibrational variational calculations with E ′ ⩽ 8000 cm −1 . The 34 S 16 O 2 and 33 S 16 O 2 line lists are compared to the experiment-based models in the HIgh-resolution TRANsmission molecular absorption database (HITRAN2012, http://www.cfa.harvard.edu/hitran/ ) and the Cologne Database for Molecular Spectroscopy, CDMS ( http://www.astro.uni-koeln.de/cdms/ ). The accuracy for computed 646 band origins is similar to what has been reported for the main isotopologue, i.e. 0.01–0.03 cm −1 for bands up to 5500 cm −1 . For rovibrational transitions, the 646 line position and intensity deviation patterns are much simpler and more self-consistent than those of the main isotopologue 626. The discrepancies are mainly found for higher K a / J transitions. 626 and 646 exhibit comparable line position and intensity agreement for lower K a / J transitions. The line position deviations for the 636 purely rotational band are parallel to those of 626 and 646, while its line intensity deviations do not show branching patterns as we found in the 626 and 646 cases. Predictions for the other minor isotopologues are expected to exhibit similar accuracy. These line lists are accurate enough to provide alternatives for missing bands of 626 and the minor isotopologues. It may significantly facilitate the laboratory spectroscopic measurement and analysis, as well as to identify these isotopologues in various astrophysical environments. [ABSTRACT FROM AUTHOR]

It is well known that the convergence of correlation energies in atomic and molecular calculations is relatively slow and that calculations aimed at high accuracy must explicitly make corrections for this. In this work we consider 1e − basis set extrapolation as a means of obtaining high accuracy. The correlation consistent basis sets of Dunning et al. have provided a convenient platform for extrapolation, with the independent variable being X = D, T, Q, 5, … . There has been much debate in the literature about the functional form to use for the extrapolation, with contention between the ‘theoretically justified’ (X + 1)−3 form and empirical forms based on exponentials or variable powers. We will dissect the theoretical justification of the (X + 1)−3 form by considering MP2 calculations on He and Ne as a function of the maximum angular momentum (⪙) in the basis using basis sets having converged radial extent. Calculations with ⪙ up to 14 were carried out for Ne. It is shown that while the asymptotic form of (⪙ + 1)−3 is clearly reached, higher order terms are very important in the range of ⪙ normally used in molecular calculations. We also use similar analysis techniques for an open shell atom and a small molecule. The functional form for the dependence of molecular properties with ⪙ is complex and it is safer to extrapolate fitting parameters than energies. [ABSTRACT FROM AUTHOR]

Using a new method, exponential parameters are developed to add to the Dunning type correlation consistent basis sets to describe core-valence correlation. Compared with existing core-valence basis sets, the new basis sets are larger, but have vastly improved convergence characteristics, especially for low-lying electronic states. The new method is very general in that it allows the explicit inclusion of electronic excited states as well as ionic states in the basis set determination. [ABSTRACT FROM AUTHOR]

We have carried out extensive ab initio calculations of the electronic structure of methane, and these results are used to compute vibrational energy levels. We include basis set extrapolations, core–valence correlation, relativistic effects, and Born–Oppenheimer breakdown terms in our calculations. Our ab initio predictions of the lowest lying levels are superb. [ABSTRACT FROM AUTHOR]

Reports on large-scale electronic structure calculations of the potential energy surface for the F + H[sub2]→HF + H reaction, with emphasis on the saddle point region. Steps involved in the energy calculation; Bend potential basis as a function of bond angle in degrees.

In order to further improve the accuracy of rovibrational IR line lists generated from the "Best Theory + Reliable High-resolution Experiment" (BTRHE) strategy from 0.01 to 0.05 cm−1, or 300–1500 MHz, to ∼10 MHz, we explore the current limits of the Data-Model-Theory synergy by examining the accuracy and consistency of existing data, then propose that "hot" bands in microwave (MW) spectra is the solution we need for future enhancements. The Ames SO 2 J = 0–20 rovibrational energy levels computed on the semi-empirically refined Ames-2 potential energy surface (PES) are fit to the Effective Hamiltonian (EH) model regularly used in the experimental infrared (IR) analysis for SO 2 isotopologues. In the fitted EH(Ames) model, the rotational constants A/B/C and all 5 quartic centrifugal distortion constants display clear, systematic, and consistent patterns along the vibrational state energy or quanta. Such consistent patterns may facilitate the vibrational assignments for MW hot bands and extract more information from high temperature MW spectra. Some EH(Expt) analyses were carried out with the lowest order Coriolis Coupling term, C 1. Their constants should not be directly compared with other EH(Expt) and EH(Ames) results. After excluding them, our δ = EH(Ames)- EH(Expt) analyses for 5 isotopologues (626, 636, 646, 628 and 828) indicates some loss of accuracy and consistency starting from vibrational states as low as 2 ν 2 or 1000 cm−1. Some EH parameters, e.g. δ K , may have relative deviations as large as 50–100% and totally lose any recognizable patterns. This simply means that current EH(Expt) models do not have the system-wide consistency we need to further refine the EH(Ames) and Ames rovibrational IR line lists. A large part of such defects are probably inherited from the limited precision of experimental line positions, i.e. 1E-3 ∼ 1E-4 cm−1, or 3–30 MHz. This is confirmed in a series of truncation tests using the Ames data. Although the EH(Ames) consistency may help identify unreliable rovibrational EH(Expt) parameters, and make reliable predictions for minor isotopologues and unobserved vibrational bands, we believe only the highly accurate "hot" MW transitions can provide real enhancements for EH(Expt) accuracy and consistency. "Hot" MW spectra should play a more significant role in the future synergy of Data, Model, and Theory in the field of rovibrational IR studies. • First systematic and quantitative study to compare the EH(Expt) and EH(BTRHE) models. • Consistency of existing SO2 EH(Expt) models needs improvement to guide the refinement of EH(BTRHE) and line lists. • The 1E-3 ∼ 1E-4 cm-1 data precision of most experimental rovibrational data hurts the EH(Expt) accuracy and consistency. • "Hot" bands in the microwave and Far-IR region are the most promising solution to improve the rovibrational IR analysis. • Including the lowest order Coriolis coupling term affects all EH constants. It makes EH models hard to compare.. [ABSTRACT FROM AUTHOR]

We compute nonadiabatic corrections for all bound and long-lived quasi-bound vibrational levels of H[sub 2][sup +] and HD[sup +] for selected rotational levels. This is done using the Bunker and Moss formalism with the correction factors computed from ab initio wave functions. The electronic wave functions are expanded in terms of nuclear centered Gaussian basis functions. The agreement with accurate calculations is very good: for H[sub 2][sup +], the root-mean-square error in the computed dissociation energies is 0.0006 cm-1, and, furthermore, most transition frequencies are predicted to within about 0.0001 cm-1. For HD[sup +], the results are not quite as good due to the uncertainties in the adiabatic correction. This paves the way for using these techniques to accurately predict the nonadiabatic effects for more complicated molecules. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In general, when computing intensities for polyatomics, one has to interpolate the dipole moment function obtained from ab initio calculations. For some high overtones of the water molecule, the computed intensities can be very sensitive to the way in which the interpolation is done. Our previous analytic representation [H. Partridge and D. W. Schwenke, J. Chem. Phys. 106, 4618 (1997)] was not adequate. We show that stable results can be obtained, and these results are in much improved agreement with experiment. We also test the importance of core electron correlation on intensities, and find the effect to be negligible. Of the existing water dipole moment functions in the literature, the present one is the most accurate. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a notational distinction between generalized moments and auxiliary functions in generalized moment expansion for observables of stochastic processes.

An accurate ab initio quartic force field for HNO has been determined using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), in conjunction with the correlation consistent polarized valence triple zeta (cc-pVTZ) basis set. Improved harmonic frequencies were determined with the cc-pVQZ basis set. Fundamental vibrational frequencies were determined using a second-order perturbation theory analysis and also using variational calculations. The N–O stretch and bending fundamentals are determined well from both vibrational analyses. The H–N stretch, however, is shown to have an unusually large anharmonic correction, and is not well determined using second-order perturbation theory. The H–N fundamental is well determined from the variational calculations, demonstrating the quality of the ab initio quartic force field. The zero-point energy of HNO that should be used in isodesmic reactions is also discussed. [ABSTRACT FROM AUTHOR]

State-to-state reaction probabilities are found to be highly final-state specific at state-selected threshold energies for the reactions O+H2→OH+H and H+H2→H2+H. The study includes initial rotational states with quantum numbers 0–15, and the specificity is especially dramatic for the more highly rotationally excited reactants. The analysis is based on accurate quantum mechanical reactive scattering calculations. Final-state specificity is shown in general to increase with the rotational quantum number of the reactant diatom, and the trends are confirmed for both zero and nonzero values of the total angular momentum. [ABSTRACT FROM AUTHOR]

We present improved techniques and well-optimized basis sets for application of the outgoing wave variational principle to calculate converged quantum mechanical reaction probabilities, and we illustrate them with calculations for the reactions D+H2→HD+H with total angular momentum J=3 and F+H2→HF+H with J=0 and 3. The optimization involves the choice of distortion potential, the grid for calculating half-integrated Green’s functions, the placement, width, and number of primitive distributed Gaussians, and the computationally most efficient partition between dynamically adapted and primitive basis functions. We present benchmark calculations with 224–1064 channels. [ABSTRACT FROM AUTHOR]

A new potential energy surface is presented for the reaction F+H2→HF+H. The regions of the surface corresponding to collinear and bent geometries in the F–H–H and H–F–H barrier regions are based on scaled external correlation (SEC) electronic structure calculations, and the F–H···H exit channel region is based on the previously developed surface No. 5. The functional form of the new surface includes dispersion forces by a double many-body expansion (DMBE), and the surface was adjusted so that the van der Waals well in the F···H–H region agrees with available experimental predictions. We have calculated stationary point properties for the new surface as well as product–valley barrier maxima of vibrationally adiabatic potential curves for F+H2→HF(v’=3)+H,F+HD→HF(v’=3)+D, and F+D2→DF(v’=4)+D. The new surface should prove useful for studying the effect on dynamics of a low, early barrier with a wide, flat bend potential, as indicated by the best available electronic structure calculations. [ABSTRACT FROM AUTHOR]

The thermal rate coefficient for the prototype reaction H+H2→H2+H with zero total angular momentum is calculated by summing, averaging, and numerically integrating state-to-state reaction probabilities calculated by time-independent quantum-mechanical scattering theory. The results are very carefully converged with respect to all numerical parameters in order to provide high-precision benchmark results for confirming the accuracy of new methods and testing their efficiency. [ABSTRACT FROM AUTHOR]