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Beginning with the seminal paper of Born and Oppenheimer (BO) [1] in 1927, the concept of the potential energy surface (PES) plays a critical role in the description, simulation, and modeling of molecular systems. It provides the basis [2] for understanding the processes associated with the nuclear motions in molecules. By going beyond the characteristic stationary points and barriers, full dimensional, accurate potential energy surfaces have a very broad range of applications in many areas of physical chemistry; for example, they provide insight into structure, reactivity, and spectroscopy of molecules.While the majority of stable structures on PES are associated with the covalent or ionic bonding [3], the regions of PES dominated by van derWaals interactions are essential for low-temperature phenomena and molecular stacking which are critical for understanding biomolecular structures involving DNA and RNA molecules [4]. Furthermore, the advances in “cold chemistry” [5] make it possible to test the theoretical predictions (see, e.g., [6]) involving very small barriers of

The use of accurate quartic force fields together with vibrational configuration interaction recently predicted gas phase fundamental vibrational frequencies of the trans-HOCO radical to within 4 cm(-1) of experimental results for the two highest frequency modes. Utilizing the same approach, we are providing a full list of fundamental vibrational frequencies and spectroscopic constants for the cis-HOCO system in both radical and anionic forms. Our predicted geometrical parameters of the cis-HOCO radical match experiment and previous computation to better than 1% deviation, and previous theoretical work agrees equally well for the anion. Correspondence between vibrational perturbation theory and variational vibrational configuration interaction for prediction of the frequencies of each mode is strong, better than 5 cm(-1), except for the torsional motion, similar to what has been previously identified in the trans-HOCO radical. Among other considerations, our results are immediately applicable to dissociative photodetachment experiments which initially draw on the cis-HOCO anion since it is the most stable conformer of the anion and is used to gain insight into the portion of the OH + CO potential surface where the HOCO radical is believed to form, and we are also providing highly accurate electron binding energies relevant to these experiments.

In the search for a full mechanism creating CO(2) from OH + CO, it has been suggested that creation of the hydroxyformyl or HOCO radical may be a necessary step. This reaction and its transient intermediate may also be responsible for the regeneration of CO(2) in such high quantities in the atmosphere of Mars. Past spectroscopic observations of this radical have been limited and a full gas phase set of the fundamental vibrational frequencies of the HOCO radical has not been reported. Using established, highly accurate quantum chemical coupled cluster techniques and quartic force fields, we are able to compute all six fundamental vibrational frequencies and other spectroscopic constants for trans-HOCO in the gas phase. These methods have yielded rotational constants that are within 0.01 cm(-1) for A(0) and 10(-4) cm(-1) for B(0) and C(0) compared with experiment as well as fundamental vibrational frequencies within 4 cm(-1) of the known gas phase experimental ν(1) and ν(2) modes. Such results lead us to conclude that our prediction of the other four fundamental modes of trans-HOCO are also quite reliable for comparison to future experimental observation, though the discrepancy for the torsional mode may be larger since it is fairly anharmonic. With the upcoming European Space Agency/NASA ExoMars Trace Gas Orbiter, these data may help to establish whether HOCO is present in the Martian sky and what role it may play in the retention of a CO(2)-rich atmosphere. Furthermore, these data may also help to clear up questions built around the fundamental chemical process of how exactly the OH + CO reaction progresses.

We report full-dimensional, ab initio potential energy and dipole moment surfaces, denoted PES and DMS, respectively, for arbitrary numbers of water monomers. The PES is a sum of 1-, 2-, and 3-body potentials which can also be augmented by semiempirical long-range higher-body interactions. The 1-body potential is a spectroscopically accurate monomer potential, and the 2- and 3-body potentials are permutationally invariant fits to tens of thousands of CCSD(T)/aug-cc-pVTZ and MP2/aug-cc-pVTZ electronic energies, respectively. The DMS is a sum of 1- and 2-body DMS, which are covariant fits to tens of thousands MP2/aug-cc-pVTZ dipole moment data. We present the details of these new 2- and 3-body potentials and then extensive applications and tests of this PES are made to the structures, classical binding energies, and harmonic frequencies of water clusters up to the 22-mer. In addition, we report the dipole moment for these clusters at various minima and compare the results against available and new ab initio calculations.

One-particle basis set extrapolation is compared with one of the new R12 methods for computing highly accurate quartic force fields (QFFs) and spectroscopic data, including molecular structures, rotational constants, and vibrational frequencies for the H(2)O, N(2)H(+), NO(2)(+), and C(2)H(2) molecules. In general, agreement between the spectroscopic data computed from the best R12 and basis set extrapolation methods is very good with the exception of a few parameters for N(2)H(+) where it is concluded that basis set extrapolation is still preferred. The differences for H(2)O and NO(2)(+) are small and it is concluded that the QFFs from both approaches are more or less equivalent in accuracy. For C(2)H(2), however, a known one-particle basis set deficiency for C-C multiple bonds significantly degrades the quality of results obtained from basis set extrapolation and in this case the R12 approach is clearly preferred over one-particle basis set extrapolation. The R12 approach used in the present study was modified in order to obtain high precision electronic energies, which are needed when computing a QFF. We also investigated including core-correlation explicitly in the R12 calculations, but conclude that current approaches are lacking. Hence core-correlation is computed as a correction using conventional methods. Considering the results for all four molecules, it is concluded that R12 methods will soon replace basis set extrapolation approaches for high accuracy electronic structure applications such as computing QFFs and spectroscopic data for comparison to high-resolution laboratory or astronomical observations, provided one uses a robust R12 method as we have done here. The specific R12 method used in the present study, CCSD(T)(R12), incorporated a reformulation of one intermediate matrix in order to attain machine precision in the electronic energies. Final QFFs for N(2)H(+) and NO(2)(+) were computed, including basis set extrapolation, core-correlation, scalar relativity, and higher-order correlation and then used to compute highly accurate spectroscopic data for all isotopologues. Agreement with high-resolution experiment for (14)N(2)H(+) and (14)N(2)D(+) was excellent, but for (14)N(16)O(2)(+) agreement for the two stretching fundamentals is outside the expected residual uncertainty in the theoretical values, and it is concluded that there is an error in the experimental quantities. It is hoped that the highly accurate spectroscopic data presented for the minor isotopologues of N(2)H(+) and NO(2)(+) will be useful in the interpretation of future laboratory or astronomical observations.

A procedure for the calculation of molecular quartic force fields (QFFs) is proposed and investigated. The goal is to generate highly accurate ab initio QFFs that include many of the so-called "small" effects that are necessary to achieve high accuracy. The small effects investigated in the present study include correlation of the core electrons (core correlation), extrapolation to the one-particle basis set limit, correction for scalar relativistic contributions, correction for higher-order correlation effects, and inclusion of diffuse functions in the one-particle basis set. The procedure is flexible enough to allow for some effects to be computed directly, while others may be added as corrections. A single grid of points is used and is centered about an initial reference geometry that is designed to be as close as possible to the final ab initio equilibrium structure (with all effects included). It is shown that the least-squares fit of the QFF is not compromised by the added corrections, and the balance between elimination of contamination from higher-order force constants while retaining energy differences large enough to yield meaningful quartic force constants is essentially unchanged from the standard procedures we have used for many years. The initial QFF determined from the least-squares fit is transformed to the exact minimum in order to eliminate gradient terms and allow for the use of second-order perturbation theory for evaluation of spectroscopic constants. It is shown that this step has essentially no effect on the quality of the QFF largely because the initial reference structure is, by design, very close to the final ab initio equilibrium structure. The procedure is used to compute an accurate, purely ab initio QFF for the H(2)O molecule, which is used as a benchmark test case. The procedure is then applied to the ground and first excited electronic states of the HO(2) (+) molecular cation. Fundamental vibrational frequencies and spectroscopic constants from these highly accurate QFFs are compared and contrasted with previous experiment and theory. It is concluded that the spectroscopic constants determined for the X(3)A(") and A(1)A(') states of HO(2) (+) are the most accurately available.

An accurate potential energy surface has been determined for methanol from ab initio potential data at the CCSD(T) level of theory with an aug-cc-pVTZ basis. The resulting potential function is valid over all twelve vibrational degrees of freedom for all near-equilibrium and torsional configurations. A torsional reaction path has been derived for this potential, from which the low-lying vibrational levels of methanol have been calculated by the reaction path version of MULTIMODE. Comparisons with experiment and other calculations are made.

We present argon predissociation vibrational spectra of the OH(-).H(2)O and Cl(-).H(2)O complexes in the 1000-1900 cm(-1) energy range, far below the OH stretching region reported in previous studies. This extension allows us to explore the fundamental transitions of the intramolecular bending vibrations associated with the water molecule, as well as that of the shared proton inferred from previous assignments of overtones in the higher energy region. Although the water bending fundamental in the Cl(-).H(2)O spectrum is in very good agreement with expectations, the OH(-).H(2)O spectrum is quite different than anticipated, being dominated by a strong feature at 1090 cm(-1). New full-dimensionality calculations of the OH(-).H(2)O vibrational level structure using diffusion Monte Carlo and the VSCF/CI methods indicate this band arises from excitation of the shared proton.

The CH5+ molecular ion is well-known for its large amplitude motions that lead to complete scrambling of the hydrogen atoms, even in the vibrational ground state. Experiments have been reported that probe the consequences of these large amplitude motions. We recently reported that quantum zero-point effects partially quench the scrambling when CH5+ is partially deuterated. Here, the consequences of this quantum localization are investigated through calculations of the low-resolution spectra of CH4D+, CHD4+, and CD5+. The spectra are obtained by convoluting stick spectra, evaluated for individual stationary points on an ab initio potential surface, multiplying them by Diffusion Monte Carlo ground state density at that stationary point, and taking the sum. The CH/D stretch regions of CH4D+ and CD5+ are red-shifted relative to CH5+, while the overall shape of the envelope remains unaffected. In contrast, for CHD4+, the shape of the spectral envelope in the CH/D stretch region differs from the other three isotopologs. These signatures of the quantum localization of the deuterium on the spectra are discussed.

The anion chemistry of the interstellar medium (ISM) has almost exclusively been limited to linear hydrocarbons and cyanocarbons. Of the hydrocarbons, only the even n C n H– chains have been detected in the ISM, and lines hypothesized to originate with b-C3H– have been conclusively linked to the corresponding cation, as originally claimed. However, no reason has yet been provided as to why other anions cannot form, and the cyclic form of C3H– is actually the lowest-energy isomer on the anion's potential energy surface. As such, this work provides the necessary rovibrational reference data for the potential detection of this anion in the ISM or related laboratory experiments. Improvements over previously calculated rovibrational spectroscopic constants are contained herein along with graphical depictions of the pure rotational spectra at 100 K, 40 K, 20 K, and 2.7 K.

The vibrations of H3O2- and D3O2- are investigated using diffusion Monte Carlo (DMC) and vibrational configuration-interaction approaches, as implemented in the program MULTIMODE. These studies use the potential surface recently developed by Huang et al. [ J. Am. Chem. Soc. 126, 5042 (2004)]. The focus of this work is on the vibrational ground state and fundamentals which occur between 100 and 3700 cm(-1). In most cases, excellent agreement is obtained between the fundamental frequencies calculated by the two approaches. This serves to demonstrate the power of both methods for treating this very anharmonic system. Based on the results of the MULTIMODE and DMC treatments, the extent and nature of the couplings in H3O2- and D3O2- are investigated.

A purely ab initio potential energy surface (PES) was refined with selected (32)S(16)O2 HITRAN data. Compared to HITRAN, the root-mean-squares error (RMS) error for all J=0-80 rovibrational energy levels computed on the refined PES (denoted Ames-1) is 0.013 cm(exp -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 296K and covers up to 8,000 cm(exp -1). Compared to the HITRAN and CDMS databases, the intensity agreement for most vibrational bands is better than 85-90%. Our predictions for (34)S(16)O2 band origins, higher energy (32)S(16)O2 band origins and missing (32)S(16)O2 IR bands have been verified by most recent experiments and available HITRAN data. We conclude that the Ames-1 PES is able to predict (32/34)S(16)O2 band origins below 5500 cm(exp -1) with 0.01-0.03 cm(exp -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.

We report a CCSD(T)/cc-pCV5Z quartic force field (QFF) and a semi-global CCSD(T)-F12b/aug-cc-pVTZ potential energy surface (PES) for singlet, cyclic C4. Vibrational fundamentals, combinations, and overtones are obtained using vibrational second-order perturbation theory (VPT2) and the vibrational configuration-interaction (VCI) approach. Agreement is within 10 cm(-1) between the VCI calculated fundamentals on the QFF and PES using the MULTIMODE (MM) program, and VPT2 and VCI results agree for the fundamentals. The agreement between VPT2-QFF and MM-QFF results is also good for the C4 combinations and overtones. The J = 1 and J = 2 rovibrational energies are reported from both VCI (MM) on the PES and VPT2 on the QFF calculations. The spectroscopic constants of (12)C4 and two C2v-symmetry, single (13)C-substituted isotopologues are presented, which may help identification of cyclic C4 in future experimental analyses or astronomical observations.

The interstellar presence of protonated nitrous oxide has been suspected for some time. Using established high-accuracy quantum chemical techniques, spectroscopic constants and fundamental vibrational frequencies are provided for the lower energy O-protonated isomer of this cation and its deuterated isotopologue. The vibrationally-averaged B0 and C0 rotational constants are within 6 MHz of their experimental values and the D(J) quartic distortion constants agree with experiment to within 3%. The known gas phase O-H stretch of NNOH(+) is 3330.91 cm(-1), and the vibrational configuration interaction computed result is 3330.9 cm(-1). Other spectroscopic constants are also provided, as are the rest of the fundamental vibrational frequencies for NNOH(+) and its deuterated isotopologue. This high-accuracy data should serve to better inform future observational or experimental studies of the rovibrational bands of protonated nitrous oxide in the interstellar medium and the laboratory.

It has been shown that rotational lines observed in the Horsehead nebula photodissociation region (PDR) are probably not caused by l-C3H+, as was originally suggested. In the search for viable alternative candidate carriers, quartic force fields are employed here to provide highly accurate rotational constants, as well as fundamental vibrational frequencies, for another candidate carrier: 1 1 A' C3H?. The ab initio computed spectroscopic constants provided in this work are, compared to those necessary to define the observed lines, as accurate as the computed spectroscopic constants for many of the known interstellar anions. Additionally, the computed D eff for C3H? is three times closer to the D deduced from the observed Horsehead nebula lines relative to l-C3H+. As a result, 1 1 A' C3H? is a more viable candidate for these observed rotational transitions. It has been previously proposed that at least C6H? may be present in the Horsehead nebular PDR formed by way of radiative attachment through its dipole-bound excited state. C3H? could form in a similar way through its dipole-bound state, but its valence excited state increases the number of relaxation pathways possible to reach the ground electronic state. In turn, the rate of formation for C3H? could be greater than the rate of its destruction. C3H? would be the seventh confirmed interstellar anion detected within the past decade and the first C n H? molecular anion with an odd n.

Very recently, molecular rotational transitions observed in the photon-dominated region of the Horsehead nebula have been attributed to l-C3H+. In an effort to corroborate this finding, we employed state-of-the art and proven high-accuracy quantum chemical techniques to compute spectroscopic constants for this cation and its isotopologues. Even though the B rotational constant from the fit of the observed spectrum and our computations agree to within 20 MHz, a typical level of accuracy, the D rotational constant differs by more than 40%, while the H rotational constant differs by three orders of magnitude. With the likely errors in the rotational transition energies resulting from this difference in D on the order of 1 MHz for the lowest observed transition (J = 4 yields 3) and growing as J increases, the assignment of the observed rotational lines from the Horsehead nebula to l-C3H+ is questionable.

We report a full-dimensional potential energy surface for H3O2-, based on fitting 66,965 ab initio electronic energies. A major feature of this potential is a barrier of roughly 200 cm-1 to internal rotation of the two hydroxyl groups about a line connecting the two oxygen atoms and the bridging hydrogen atom. The potential is used in calculations of vibrational energies, performed with the "Reaction Path" version of the code "MULTIMODE". The results are compared to recent infrared messenger experiments and are used to propose interpretations of the experimental results.

Ammonia (NH3) was the first polyatomic molecule observed in the ISM. Its importance in interstellar molecules is only second to CO because its rovibrational spectroscopic signature can be used very effectively at deducing the conditions of the interstellar environment such as temperature and density, and because it is found in so many different interstellar objects in a wide temperature range. However, experimental determination of NH3 IR spectra is extremely difficult due to the large-amplutide inversion vibration, and the existing HITRAN2008 database for NH3 is limited in temperature, coverage, completeness, and accuracy. With rapid progress in theoretical chemistry and computational resources, now we are able to generate a highly reliable/accurate IR line list of NH3 (and its isotopologues) for astronomical studies. Exact quantum rovibrational computations on an empirically refined potential energy surface (with nonadiabatic corrections included) have achieved accuracies of 0.02-0.05 cm−1 (for line position) and better than 85-95% (for line intensity) for both NH3 and 15NH3 spectra. The unique feature of our work is that our predictions are essentially as accurate as reproducing existing measurements, suitable for synthetic simulation of various astrophysical environments or objects. The reliabilty and accuracy of our predictions for missing bands and higher energies computed on HSL-2 (Fig. 1) have been proved by the most recent high-resolution experiments and extended up to 7000 cm−1. See Huang et al. 2008, Huang et al. 2011, & Sung et al. 2012 for more details.

Highly correlated ab initio quartic force field (QFFs) are used to calculate the equilibrium structures and predict the spectroscopic parameters of three HC2N isomers. Specifically, the ground state quasilinear triplet and the lowest cyclic and bent singlet isomers are included in the present study. Extensive treatment of correlation effects were included using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T). Dunning s correlation-consistent basis sets cc-pVXZ, X=3,4,5, were used, and a three-point formula for extrapolation to the one-particle basis set limit was used. Core-correlation and scalar relativistic corrections were also included to yield highly accurate QFFs. The QFFs were used together with second-order perturbation theory (with proper treatment of Fermi resonances) and variational methods to solve the nuclear Schr dinger equation. The quasilinear nature of the triplet isomer is problematic, and it is concluded that a QFF is not adequate to describe properly all of the fundamental vibrational frequencies and spectroscopic constants (though some constants not dependent on the bending motion are well reproduced by perturbation theory). On the other hand, this procedure (a QFF together with either perturbation theory or variational methods) leads to highly accurate fundamental vibrational frequencies and spectroscopic constants for the cyclic and bent singlet isomers of HC2N. All three isomers possess significant dipole moments, 3.05D, 3.06D, and 1.71D, for the quasilinear triplet, the cyclic singlet, and the bent singlet isomers, respectively. It is concluded that the spectroscopic constants determined for the cyclic and bent singlet isomers are the most accurate available, and it is hoped that these will be useful in the interpretation of high-resolution astronomical observations or laboratory experiments.

Recently, we reported ab initio quartic force fields (QFFs) for the cyclic and linear forms of the C3H3 + molecular cation, referred to as c-C3H3 + and l-C3H3 +. These were computed using high levels of theory. Specifically the singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations, CCSD(T), was used in conjunction with extrapolation to the one-particle basis set limit, and corrections for scalar relativity and core correlation were included. In the present study, we use these QFFs to compute highly accurate fundamental vibrational frequencies and other spectroscopic constants for the c-13CC2H3 +, c-C3H2D+, c-13CC2H2D+ isotoplogues of c-C3H3 +, and the H2CCCD+, HDCCCH+, H2 13CCCH+, H2C13CCH+, and H2CC13CH+ isotopologues of l-C3H3 +. Improvements in ab intitio methods have now made it possible to identify small molecules in an astronomical observation without the aid of high-resolution experimental data. We also report dipole moment values and show that the above-mentioned cyclic isotopologues have values of 0.094, 0.225, and 0.312 D, respectively, while the l-C3H3 + isotopologues have values that range between 0.325 and 0.811 D. Thus, it is hoped that the highly accurate spectroscopic constants and data provided herein for the 13C and deuterium isotopologues of the cyclic and linear forms of C3H3 + will enable their identification in astronomical observations from the Herschel Space Observatory, the Stratospheric Observatory for Infrared Astronomy, the Atacama Large Millimeter Array, and in the future, the James Webb Space Telescope.

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 (14)NH(3), new assignments for a few ν(3) + ν(4) band transitions and energy levels are suggested, and discrepancies between computed and HITRAN energy levels in the 2ν(4) band are analyzed (2ν(4) is the most difficult band below 5000 cm(-1)). New assignments are suggested for existing or missing 2ν(4) levels. Several new vibrational bands are identified from existing, unassigned HITRAN data, including 2ν(2) + ν(4), (ν(3) + ν(4)) -A(')∕A("), ν(1) + 2ν(2), and 2ν(2) + 2ν(4). The strong mixing between the 2ν(4) and 2ν(2) + ν(4) bands is carefully examined and found to be the source of the difficulties in the experimental modeling of 2ν(4). 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 4ν(2) 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 (15)NH(3), excellent agreement is found for the available ν(2) and ν(3) + ν(4)(E) transition energies, but significant deficiencies are shown for HITRAN levels and several corrections are suggested. The (15)N isotopic effects are presented for the J = 0-6 levels of 13 HITRAN bands. For (14)ND(3), we reproduce the pure rotational inversion spectra line frequencies with an accuracy similar to that for (14)NH(3). 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 (14)ND(3) 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.

In this work, we build upon our previous work on the theoretical spectroscopy of ammonia, NH(3). 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 (14)NH(3) 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 (15)NH(3) 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 (15)NH(3) energy levels determined from a model of the experimental data.

The full dimensional rate constant reported by Huarte-Larranaga and Manthe for the H+CH4→H2+CH3 reaction [Huarte-Larranaga and Manthe, J. Chem. Phys. 113, 5115 (2000)] is corrected by using an accurate vibrational partition function for CH4 instead of the harmonic normal-mode one used by them. The correction is shown to be substantial over the temperature range considered by Huarte-Larranaga and Manthe.

One-particle basis set extrapolation is compared with one of the new R12 methods for computing highly accurate quartic force fields (QFFs) and spectroscopic data, including molecular structures, rotational constants, and vibrational frequencies for the H2O, N2H+, NO2+, and C2H2 molecules. In general, agreement between the spectroscopic data computed from the best R12 and basis set extrapolation methods is very good with the exception of a few parameters for N2H+ where it is concluded that basis set extrapolation is still preferred. The differences for H2O and NO2+ are small and it is concluded that the QFFs from both approaches are more or less equivalent in accuracy. For C2H2, however, a known one-particle basis set deficiency for C–C multiple bonds significantly degrades the quality of results obtained from basis set extrapolation and in this case the R12 approach is clearly preferred over one-particle basis set extrapolation. The R12 approach used in the present study was modified in order to obtain hi...

We report the properties of two novel transition states of the bimolecular hydrogen exchange reaction in the water dimer, based on an ab initio water dimer potential [A. Shank et al., J. Chem. Phys. 130, 144314 (2009)]. The realism of the two transition states is assessed by comparing structures, energies, and harmonic frequencies obtained from the potential energy surface and new high-level ab initio calculations. The rate constant for the exchange is obtained using conventional transition state theory with a tunneling correction. We employ a one-dimensional approach for the tunneling calculations using a relaxed potential from the full-dimensional potential in the imaginary-frequency normal mode of the saddle point, Q(im). The accuracy of this one-dimensional approach has been shown for the ground-state tunneling splittings for H and D-transfer in malonaldehyde and for the D+H(2) reaction [Y. Wang and J. M. Bowman, J. Chem. Phys. 129, 121103 (2008)]. This approach is applied to calculate the rate constant for the H(2)O+H(2)O exchange and also for H(2)O+D(2)O→2HOD. The local zero-point energy is also obtained using diffusion Monte Carlo calculations in the space of real-frequency-saddle-point normal modes, as a function of Q(im).

A series of high-quality, purely ab initio, quartic force fields (QFFs), computed using a procedure we recently proposed, is reported for NH2− and CCH−. The singles and doubles coupled-cluster method with a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T), was used with TZ, QZ, and 5Z quality basis sets and was combined with extrapolation to the one-particle basis-set limit, core-correlation effects, scalar relativistic effects, and higher-order correlation effects to yield accurate QFFs. A “best-guess” reference geometry was determined at the CCSD(T)/5Z level of theory. Analytical transformation removes nonzero gradients to facilitate a second-order perturbation theory spectroscopic analysis. The QFF is transformed into Morse/cosine coordinates in order to perform exact vibrational configuration interaction computations. Equilibrium structures, vibrational frequencies, rotational constants, and selected spectroscopic constants are reported in comparison with experim...

A global potential energy surface (PES) that includes short and long range terms has been determined for the NH(3) 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

Predissociation spectra of the H(5)O(2) (+)RG(n)(RG = Ar,Ne) cluster ions are reported in energy regions corresponding to both the OH stretching (3350-3850 cm(-1)) and shared proton (850-1950 cm(-1)) vibrations. The two free OH stretching bands displayed by the Ne complex are quite close to the band origins identified earlier in bare H(5)O(2) (+) [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)], indicating that the symmetrical H(5)O(2) (+) "Zundel" ion remains largely intact in H(5)O(2) (+)Ne. The low-energy spectrum of the Ne complex is simpler than that observed previously for H(5)O(2) (+)Ar, and is dominated by two sharp transitions at 928 and 1047 cm(-1), with a weaker feature at 1763 cm(-1). The H(5)O(2) (+)Ar(n),n = 1-5 spectra generally exhibit complex band structures reflecting solvent-induced symmetry breaking of the Zundel core ion. The extent of solvent perturbation is evaluated with electronic structure calculations, which predict that the rare gas atoms should attach to the spectator OH groups of H(5)O(2) (+) rather than to the shared proton. In the asymmetric complexes, the shared proton resides closer to the more heavily solvated water molecule, leading to redshifts in the rare gas atom-solvated OH stretches and to blueshifts in the shared proton vibrations. The experimental spectra are compared with recent full-dimensional vibrational calculations (diffusion Monte Carlo and multimode/vibrational configuration interaction) on H(5)O(2) (+). These results are consistent with assignment of the strong low-energy bands in the H(5)O(2) (+)Ne spectrum to the vibration of the shared proton mostly along the O-O axis, with the 1763 cm(-1) band traced primarily to the out-of-phase, intramolecular bending vibrations of the two water molecules.