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  • 1
    Electronic Resource
    Electronic Resource
    Ab initio calculation of the rotation–vibration energy levels of H3+ and its isotopomers to spectroscopic accuracy (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 5056-5064 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Surfaces are fitted to the Born–Oppenheimer potential energy, electronic relativistic correction and adiabatic correction data calculated ab initio by Cencek et al. [J. Chem. Phys. 108, 2831 (1998)]. These surfaces are used in calculations of the rotation–vibration energy levels of H3+, H2D+, D2H+, and D3+. Nonadiabatic corrections to the Born–Oppenheimer approximation are introduced following models developed for diatomics which involve the use of isotopomer independent scaled vibrational reduced masses. It is shown that for triatomics this approach leads to an extra term in the nuclear motion Hamiltonian. Our final calculations reproduce the known spectroscopic data for H3+ and its isotopomers to within a few hundredths of a cm−1. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478404
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  • 2
    Electronic Resource
    Electronic Resource
    Calculated high-temperature partition function and related thermodynamic data for H2〈sup ARRANGE="STAGGER"〉16O (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 7197-7204 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The partition function, Q, of water is calculated by explicit summation of ∼200 000 vibration–rotation levels computed using variational nuclear motion calculations. Temperatures up to 6000 K are studied. Estimates are obtained for the heat capacity (Cp), the Gibbs enthalpy factor (gef), the Helmholtz function (hcf), and the entropy (S) of gas-phase water as a function of temperature. To get converged results at higher temperatures it is necessary to augment the accurate list of energy levels. This is done using estimates for all the vibrational band origins to dissociation and rotational levels calculated using Padé approximants. The widely used method of computing the internal partition function as the product of vibrational and rotational partition functions is tested and found to overestimate the partition function by up to 10%. The present estimates of Q(T), Cp(T), gef(T), hcf(T), and S(T) are probably the most accurate available for water at temperatures, T, above 2000 K. Errors, as a function of temperature, are estimated in each case. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477400
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  • 3
    Electronic Resource
    Electronic Resource
    Calculation of the rotation–vibration states of water up to dissociation (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 10885-10892 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present rotation–vibrational levels of water up to the dissociation limit using two recent, global potential energy surfaces. These calculations are performed using our recently developed discrete variable representation (DVR) based parallel code (PDVR3D), which runs on computers with massively parallel processors. Variational tests on the convergence of these results show convergence within 0.5 cm−1. Analysis of the highest wave functions for the vibrational energy levels are also shown. Tests on previous calculations performed using conventional computers suggest that convergence for high-lying rotationally excited states is not as good as claimed. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476519
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  • 4
    Electronic Resource
    Electronic Resource
    Asymmetric adiabatic correction to the rotation–vibration levels of H2D+ and D2H+ (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 9322-9326 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Calculations on H2D+ and D2H+ have shown that the energy levels of these asymmetric isotopomers of H3+ cannot be reproduced using effective potential energy surfaces with D3h symmetry. It is shown that for these ions the adiabatic correction to the Born–Oppenheimer approximation has an asymmetric component which can be expressed as a mass-independent surface multiplied by a mass factor. An expression for this function is obtained from ab initio calculations. Use of this adiabatic correction is found to resolve the discrepancy with the levels of H2D+ and D2H+. The ab initio calculations reported reproduce the observed H2D+ transitions with an average error (obs−calc) of −8 MHz for the rotational transitions, −0.06 cm−1 for the ν1 band, −0.13 cm−1 for ν2, and −0.19 cm−1 for ν3. These errors are nearly constant for all transitions within a vibrational band. This gives a very accurate ab initio framework for predicting unobserved transition frequencies. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468799
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  • 5
    Electronic Resource
    Electronic Resource
    A spectroscopically determined potential energy surface for the ground state of H216O: A new level of accuracy (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7651-7657 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The potential energy function for the electronic ground state of the water molecule has been obtained by fitting rotation-vibration term values involving J≤14 for 24 vibrational states of H216O together with 25 additional vibrational term values belonging to higher excited states. The fitting was carried out by means of an exact kinetic energy Hamiltonian. It was found that the differences between the exact kinetic energy calculations and calculations with the morbid program (i.e., calculations with an approximate kinetic energy operator) depend only very slightly on the parameters of the potential. This fact allowed us to make an inexpensive fitting using the morbid approach and still get the accuracy obtainable with the exact kinetic energy Hamiltonian. The standard deviation for 1600 term values was 0.36 cm−1. For 220 ground state energy levels the standard deviation was 0.03 cm−1. With the fitted potential, calculations of term values with J≤35 were carried out. This showed the excellent predictive power of the new potential. For the J=20 term values in the vibrational ground state, the deviations from experiment are typically below 0.2 cm−1. The discrepancy for the observed level with the highest Ka value, JKaKc = 20200, is only 0.008 cm−1. The calculated term value for the observed level with the highest J, 35035, deviates 0.1 cm−1 from experiment. Because of the level of accuracy achieved in these calculations, we can for the first time demonstrate the breakdown of the Born–Oppenheimer approximation for the water molecule. The high Ka level calculations allow us to show that the rotational energy level structure in water is at least of a very different nature than the fourfold cluster structures observed for H2Se and calculated for H2S, H2Se, and H2Te.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468258
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  • 6
    Electronic Resource
    Electronic Resource
    Predicted vibration–rotation levels of H2He+ and its isotopomers (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 6648-6652 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Variational bound state nuclear motion calculations are performed on a fitted [Joseph and Sathyamurthy, J. Chem. Phys. 86, 704 (1987)] ab initio surface for HeH+2. The linear 4HeH+2 molecule is predicted to have an He–H+2 stretch fundamental of 732 cm−1 with the first J=0 excited bend at 960 cm−1. This isotopomer has no bound excited states of the H+2 vibrational mode. Calculations with J=1 and J=2 predict a rotational constant of 4.0 cm−1 for this system. Results are also presented for all the isotopomers containing D and 3He. The spectrum of this molecule is a good candidate for laboratory or astrophysical observation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453399
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  • 7
    Electronic Resource
    Electronic Resource
    On the vibrational Born–Oppenheimer separation scheme for molecules with regular and chaotic states (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 6210-6217 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quantum mechanical calculations on KCN, LiCN, and ArHCl with two-dimensional (2D) potentials show that the Born–Oppenheimer separation scheme with the slow coordinate as the adiabatic variable is reliable for regular states but not for chaotic states. Three-dimensional (3D) calculations on KCN and ArHCl, where the coupling of CN or HCl motion with the other coordinates is introduced through the kinetic part of the Hamiltonian, demonstrate the existence of an extra constant of motion even for high excitations of the diatom. In 3D, cuts through the wave function show regular nodal patterns across the regular and chaotic coordinates. However, the number of nodes displaced by the chaotic coordinate varies for different values of the third constant coordinate. It is suggested that these plots provide a useful diagnostic aid for multidimensional irregular states. The adiabatic separation of regular motion gives results in excellent agreement with fully coupled 3D calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450764
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  • 8
    Electronic Resource
    Electronic Resource
    Rovibrational spectrum of the excited potential energy surface of He+H2 (B 1Σu+) (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 2163-2164 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448355
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  • 9
    Electronic Resource
    Electronic Resource
    Quantum and classical vibrational chaos in floppy molecules (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 800-809 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Classical and quantum mechanical calculations on the vibrational motions of LiCN using a realistic potential are presented. These, together with recent results for KCN, are analyzed using indicators that have been proposed for identifying quantum chaos. These are nodal structure, dominant coefficient, overlapping avoided crossings, second differences, and spectral distribution. We find an early onset of chaos in LiCN and KCN with good agreement between the indicators. Localized quasiperiodic trajectories and regular states are found well into the chaotic region for both isomers of LiCN. LiCN is a weakly coupled system in contrast to strongly coupled KCN. The utility of the indicators is discussed in the light of these results. The link between floppy molecules and chaos suggests that floppy systems are suitable for the experimental investigation of vibrational chaos.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448506
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  • 10
    Electronic Resource
    Electronic Resource
    Calculations of rotation–vibration states with the z axis perpendicular to the plane: High accuracy results for H3+ (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 7564-7573 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method of calculation of rotation–vibration states for a general triatomic that places the body-fixed z axis perpendicular to the plane of the molecule is implemented within a discrete variable representation (DVR) for the vibrational motion. Calculations are presented for water and H3+. For H3+ the new method improves on previous high accuracy ab initio treatments of the rotation–vibration energies of the molecule both in accuracy and the range of rotational states that can be treated. Reliable treatment of quasilinear geometries means that the method is also promising for treating very highly excited states. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1464540
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