Global Analytical Potential Energy Surface for the Electronic Ground State of NH 3 from High Level ab Initio Calculations (original) (raw)
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Vibrational energies for NH3 based on high level ab initio potential energy surfaces
Chemical Physics - CHEM PHYS, 2002
Ab initio coupled cluster calculations with single and double substitutions and a perturbative treatment of connected triple substitutions ͓CCSD͑T͔͒ have been carried out to generate six-dimensional ͑6D͒ potential energy surfaces ͑PES͒ and dipole moment surfaces ͑DMS͒ for the electronic ground state of ammonia. Full 6D-PES and 6D-DMS ͑14400 points͒ were computed with the augmented correlation-consistent triple-zeta basis ͑aug-cc-pVTZ͒. For a selected number of points ͑420 in C 3v symmetry and 1260 in lower symmetry͒, more accurate energies ͑CBSϩ͒ were obtained by extrapolating the CCSD͑T͒ results for the aug-cc-pVXZ (XϭT,Q,5) basis sets to the complete basis set limit and adding corrections for core-valence correlation and relativistic effects. Two procedures were investigated to enhance the quality of the 6D-PES from CCSD͑T͒/ aug-cc-pVTZ by including the CBSϩ data points. The resulting 6D-PES were represented by analytical functions involving Morse variables for the stretches, symmetry-adapted bending coordinates, and a specially designed inversion coordinate ͑up to 76 fitted parameters, rms deviations of about 5 cm Ϫ1 for 14 400 ab initio data points͒. For these analytical surfaces, vibrational energies were calculated with a newly developed computer program using a variational model that employs an Eckart-frame kinetic energy operator. Results are presented and compared to experiment for the vibrational band centers of NH 3 and its isotopomers up to around 15 000 cm Ϫ1 . For our best 6D-PES, the term values of the fundamentals are reproduced with rms deviations of 4.4 cm Ϫ1 (NH 3 ) and 2.6 cm Ϫ1 ͑all isotopomers͒, the maximum deviation being 7.9 cm Ϫ1 .
Vibrational energies for NH[sub 3] based on high level ab initio potential energy surfaces
The Journal of Chemical Physics, 2002
Ab initio coupled cluster calculations with single and double substitutions and a perturbative treatment of connected triple substitutions ͓CCSD͑T͔͒ have been carried out to generate six-dimensional ͑6D͒ potential energy surfaces ͑PES͒ and dipole moment surfaces ͑DMS͒ for the electronic ground state of ammonia. Full 6D-PES and 6D-DMS ͑14400 points͒ were computed with the augmented correlation-consistent triple-zeta basis ͑aug-cc-pVTZ͒. For a selected number of points ͑420 in C 3v symmetry and 1260 in lower symmetry͒, more accurate energies ͑CBSϩ͒ were obtained by extrapolating the CCSD͑T͒ results for the aug-cc-pVXZ (XϭT,Q,5) basis sets to the complete basis set limit and adding corrections for core-valence correlation and relativistic effects. Two procedures were investigated to enhance the quality of the 6D-PES from CCSD͑T͒/ aug-cc-pVTZ by including the CBSϩ data points. The resulting 6D-PES were represented by analytical functions involving Morse variables for the stretches, symmetry-adapted bending coordinates, and a specially designed inversion coordinate ͑up to 76 fitted parameters, rms deviations of about 5 cm Ϫ1 for 14 400 ab initio data points͒. For these analytical surfaces, vibrational energies were calculated with a newly developed computer program using a variational model that employs an Eckart-frame kinetic energy operator. Results are presented and compared to experiment for the vibrational band centers of NH 3 and its isotopomers up to around 15 000 cm Ϫ1 . For our best 6D-PES, the term values of the fundamentals are reproduced with rms deviations of 4.4 cm Ϫ1 (NH 3 ) and 2.6 cm Ϫ1 ͑all isotopomers͒, the maximum deviation being 7.9 cm Ϫ1 .
Nearly 4800 features of ammonia between 6300 and 7000 cm À 1 with intensities Z 4 Â 10 À 24 cm À 1 /(molecule Á cm À 2 ) at 296 K were measured using 16 pure NH 3 spectra recorded at various temperatures (296-185 K) with the McMath-Pierce Fourier Transform Spectrometer at Kitt Peak National Observatory, AZ. The line positions and intensities were retrieved by fitting individual spectra based on a Voigt line shape profile and then averaging the values to form the experimental linelist. The integrated intensity of the region was 4.68 Â 10 À 19 cm À 1 /(molecule Á cm À 2 ) at 296 K. Empirical lower state energies were also estimated for 3567 absorption line features using line intensities retrieved from 10 spectra recorded at gas temperature between 185 and 233 K. Finally, using Ground State Combination Differences (GSCDs) and the empirical lower state energy estimates, the quantum assignments were determined for 1096 transitions in the room temperature linelist, along with empirical upper state energies for 434 levels. The assignments correspond to seven vibrational states, as confirmed from recent ab initio calculations. The resulting composite database of 14 NH 3 line parameters will provide experimental constraints to ab initio calculations and support remote sensing of gaseous bodies including the atmospheres of Earth, (exo)planets, brown dwarfs, and other astrophysical environments.
The Journal of Chemical Physics, 2011
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-meansquare 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 . 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 .
Chemical Physics Letters, 2002
Results of six-dimensional variational calculations of vibrational energy levels are presented for ammonia using a Hamiltonian expressed in curvilinear internal bond coordinates. A two-dimensional potential energy surface, which was introduced in a preliminary study on the inversion motion, is combined with a surface by Martin et al. Both surfaces are calculated at the aug-cc-pVTZ/CCSD(T) ab initio level. The exact kinetic energy operator is an enlargement of the one used in the previous two-dimensional calculations. Eigenvalues are computed variationally using successive basis set contractions for some symmetric and asymmetric isotopomers of ammonia. Ó
A Variationally Computed T = 300 K Line List for NH 3 †
The Journal of Physical Chemistry A, 2009
Calculations are reported on the rotation-vibration energy levels of ammonia with associated transition intensities. A potential energy surface obtained from coupled cluster CCSD(T) calculations and subsequent fitting against experimental data is further refined by a slight adjustment of the equilibrium geometry, which leads to a significant improvement in the rotational energy level structure. A new accurate ab initio dipole moment surface is determined at the frozen core CCSD(T)/aug-cc-pVQZ level. The calculation of an extensive ammonia line list necessitates a number of algorithmic improvements in the program TROVE that is used for the variational treatment of nuclear motion. Rotation-vibration transitions for 14 NH 3 involving states with energies up to 12000 cm -1 and rotational quantum number J ) 20 are calculated. This gives 3.25 million transitions between 184400 energy levels. Comparisons show good agreement with data in the HITRAN database but suggest that HITRAN is missing significant ammonia absorptions, particularly in the nearinfrared. † Part of the "Walter Thiel Festschrift".
International Journal of Quantum Chemistry, 2004
The influence of basis set superposition error (BSSE) in optimized geometries, force constants and intermolecular harmonic vibrational frequencies of the ammonia–ammonia dimer have been studied both at the Hartree–Fock and correlated (second-order Møller–Plesset perturbation theory) levels of theory using several different basis sets as (6-31G, 6-311G, 6-31++G, 6-311++G, 6-31G(d,p), 6-311G(d,p), 6-31++G(d,p) 6-311++G(d,p) and 6-311++G(2d,2p)). The widely used a posteriori Boys–Bernardi “counterpoise” (CP) correction scheme has been compared with the a priori method utilizing the chemical Hamiltonian approach (CHA). The results show that practically there is no difference between these two methods, so the a priori CHA scheme can be considered as an ultimate solution of the BSSE problem. It is also concluded that the BSSE influence is very significant, so removing this effect is very important. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004
The Journal of Chemical Physics, 2010
We present four-dimensional ab initio potential energy surfaces for the three different spin states of the NH( 3 Σ − ) -NH( 3 Σ − ) complex. The potentials are partially based on the work of Dhont et al. [J. Chem. Phys. 123, 184302 (2005)]. The surface for the quintet state is obtained at the RCCSD(T)/aug-cc-pVTZ level of theory and the energy differences with the singlet and triplet states are calculated at the CASPTn/aug-cc-pVTZ (n = 2, 3) level of theory. The ab initio potentials are fitted to coupled spherical harmonics in the angular coordinates, and the long range is further expanded as a power series in 1/R. The RCCSD(T) potential is corrected for a sizeconsistency error of about 0.5 × 10 −6 E h prior to fitting. The long-range coefficients obtained from the fit are found to be in good agreement with first and second-order perturbation theory calculations.
Journal of Molecular Structure: THEOCHEM, 1993
The dependence of the calculated molecular properties of ammonia on the atomic basis set used was studied systematically. The molecular properties considered were: some one-electron properties (dipole moment, quadrupole moment and electric field gradient), geometry, harmonic vibrational wavenumbers and intensities, and the inversion barrier. These properties were computed at various levels of theory (restricted Hartree-Fock (RHF), second-order Moller-Plesset (MP2), configuration interaction with single and double excitation (CISD), complete active space selfconsistent field (CASSCF), coupled pair function (CPF) and coupled cluster using double substitution (CCD)) using different quality atomic basis sets (double-zeta plus polarization (DZ + P), triple-zeta plus polarization (TZ + P) and atomic natural orbital (ANO)). The dependence of the one-electron properties on the inversion was explored at the CISD/ANO level of theory. Finally, the vibrational spectrum of the inversion for NH,, NDs and NT, was calculated using a high-quality potential function for ammonia.