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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 12C4 and two C2v-symmetry, single 13C-substituted isotopologues are presented, which may help identification of cyclic C4 in future experimental analyses or astronomical observations. [ABSTRACT FROM AUTHOR]

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 DJ 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. [ABSTRACT FROM AUTHOR]

Highly correlated ab initio quartic force fields (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 as 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 (PT) (with proper treatment of Fermi resonances) and variational methods to solve the nuclear Schrödinger 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 PT). On the other hand, this procedure (a QFF together with either PT 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, 3.05 D, 3.06 D, and 1.71 D, 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. [ABSTRACT FROM AUTHOR]

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 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 N2H+ and NO2+ 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 14N2H+ and 14N2D+ was excellent, but for 14N16O2+ 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 N2H+ and NO2+ will be useful in the interpretation of future laboratory or astronomical observations. [ABSTRACT FROM AUTHOR]

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 H2O molecule, which is used as a benchmark test case. The procedure is then applied to the ground and first excited electronic states of the HO2+ 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 X3A″ and A1A′ states of HO2+ are the most accurately available. [ABSTRACT FROM AUTHOR]

A very accurate ab initio quartic force field for CH4 and its isotopomers is presented. The quartic force field was determined with the singles and doubles coupled-cluster procedure that includes a quasiperturbative estimate of the effects of connected triple excitations, CCSD(T), using the correlation consistent polarized valence triple zeta, cc-pVTZ, basis set. Improved quadratic force constants were evaluated with the correlation consistent polarized valence quadruple zeta, cc-pVQZ, basis set. Fundamental vibrational frequencies are determined using second-order perturbation theory anharmonic analyses. All fundamentals of CH4 and isotopomers for which accurate experimental values exist and for which there is not a large Fermi resonance, are predicted to within ±6 cm-1. It is thus concluded that our predictions for the harmonic frequencies and the anharmonic constants are the most accurate estimates available. It is also shown that using cubic and quartic force constants determined with the correlation consistent polarized double zeta, cc-pVDZ, basis set in conjunction with the cc-pVQZ quadratic force constants and equilibrium geometry leads to accurate predictions for the fundamental vibrational frequencies of methane, suggesting that this approach may be a viable alternative for larger molecules. Using CCSD(T), core correlation is found to reduce the CH4 re by 0.0015 Å. Our best estimate for re is 1.0862±0.0005 Å. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Assesses the spectroscopic quality of quartic force field for acetylene. Details on coupled cluster with all single and double substitutions and quasiperturbative inclusion of connected Triple excitations method; Bending modes of carbon-carbon multiple bonds; Computations on quartic force field and harmonic frequencies.

Discusses the use of the singles and doubles coupled-cluster method that includes a perturbational correction for connected triple excitations, in conjunction with several one-particle basis sets to determine quartic force fields for hydroxide. Rovibrational spectroscopic constants; Experimental observation of hot bands.

The CCSD(T) method has been used to compute a highly accurate quartic force field and fundamental frequencies for all 14N and 15N isotopomers of the high energy density material TdN4. The computed fundamental frequencies show beyond doubt that the bands observed in a matrix isolation experiment by Radziszewski and coworkers [Chem. Phys. Lett. 328 (2000) 227] are not due to different isotopomers of TdN4. The most sophisticated thermochemical calculations to date yield a 2N2→N4 heat of reaction of 182.22±0.5 kcal/mol at 0 K (180.64±0.5 at 298 K). It is hoped that the data reported herein will aid in the ultimate detection of TdN4. [Copyright &y& Elsevier]

The spectroscopic constants and vibrationalfrequencies for the1 3A′ states of HNC, DNC, HOC+, and DOC+are computed and discussed in this work. The reliable CcCRquartic force field based on high-level coupled cluster ab initioquantum chemical computations is exclusively utilized to provide theanharmonic potential. Then, second-order vibrational perturbationtheory and vibrational configuration interaction methods are employedto treat the nuclear Schrödinger equation. Second-order perturbationtheory is also employed to provide spectroscopic data for all moleculesexamined. The relationship between these molecules and the corresponding1 3A′ HCN and HCO+isomers is furtherdeveloped here. These data are applicable to laboratory studies involvingformation of HNC and HOC+as well as astronomical observationsof chemically active astrophysical environments. [ABSTRACT FROM AUTHOR]

A semiglobal potential energy surface (PES) and quartic force field (QFF) based on fitting high-level electronic structure energies are presented to describe the structures and spectroscopic properties of NNHNN+. The equilibrium structure of NNHNN+ is linear with the proton equidistant between the two nitrogen groups and thus of D∞h symmetry. Vibrational second-order perturbation theory (VPT2) calculations based on the QFF fails to describe the proton "rattle" motion, i.e., the antisymmetric proton stretch, due to the very flat nature of PES around the global minimum but performs properly for other modes with sharper potential wells. Vibrational self-consistent field/virtual state configuration interaction (VSCF/VCI) calculations using a version of MULTIMODE without angular momentum terms successfully describe this motion and predict the fundamental to be at 759 cm-1. This is in good agreement with the value of 746 cm-1 from a fixed-node diffusion Monte Carlo calculation and the experimental Ar-tagged result of 743 cm-1. Other VSCF/VCI energies are in good agreement with other experimentally reported ones. Both double-harmonic intensity and rigorous MULTIMODE intensity calculations show the proton-transfer fundamental has strong intensity. [ABSTRACT FROM AUTHOR]

Highlyaccurate, coupled-cluster-based quartic force fields (QFFs) have beenemployed recently to provide spectroscopic reference for a myriadof molecules. Here, we are extending the same approach to providevibrational and rotational spectroscopic reference data for the sulfuranalogues of HOCO, HSCO, and HOCS, in both the cis and trans conformationsas well as the D and 34S isotopologues of each system.The resulting energies corroborate previous computations showing that trans-HSCO is the lowest-energy isomer for this system.The vibrational frequencies are computed with both second-order vibrationalperturbation theory (VPT2) and vibrational configuration interaction(VCI) methods. The VPT2 and VCI QFF frequencies largely agree withone another to better than 5.0 cm–1(often betterthan 1.0 cm–1) and are also consistent with thetype of behavior exhibited in previous studies. As such, the referencedata provided here should assist in analysis of environments in whichthese sulfur systems may be found, including the interstellar medium,combustion flames, or laboratory simulations of either. [ABSTRACT FROM AUTHOR]

Highlights: [•] Quartic Force Fields (QFFs) yield highly accurate spectra including rotational structure. [•] Infrared intensities (IR) are well determined using a CCSD(T) Dipole Moment Surface (DMS). [•] Purely ab initio IR intensities agree extremely well with high-resolution experiment. [•] Purely ab initio transition energies agree very well with high-resolution experiment. [•] Ab initio purely rotational and rovibrational line lists should aid astronomers to assign lines. [Copyright &y& Elsevier]

Highlights: [•] Quartic force fields (QFFs) yield highly accurate vibrational spectra using variational methods. [•] The QFF needs to be transformed into a coordinate system with correct limiting behavior. [•] For stretches, a Morse coordinate is defined and shown to perform very well. [•] For bends and other motions, cosine or sine coordinates are most often used. [•] QFFs can maintain exact symmetry for non-Abelian groups, yielding exactly degenerate vibrations. [Copyright &y& Elsevier]