Approximate calculation of anharmonic densities of vibrational states for very large molecules

Schmatz, Stefan

doi:10.1016/j.chemphys.2008.01.020

Cited by 14 publications

(17 citation statements)

References 68 publications

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“…For the simulations reported here the vibrational energy of the transition state is less than the vibrational energy E total of CH 4 * and, thus, the anharmonic correction f anh,ρ ( E ) = ρ anh ( E )/ρ h ( E ) is expected to be more important than f anh, N ≠ ( E ). As determined by three research groups, − f anh,ρ ( E ) is 2 at the CH 4 dissociation energy of ∼110 kcal/mol, giving f anh ( E ) = 0.5 at this energy if f anh, N ≠ ( E ) is assumed to be unity. Such a value is consistent with the value of 0.4 above for E total of 120–130 kcal/mol.…”

Section: Resultsmentioning

confidence: 99%

Zero-Point Energy Constraint for Unimolecular Dissociation Reactions. Giving Trajectories Multiple Chances To Dissociate Correctly

Paul

Hase

2016

J. Phys. Chem. A

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A zero-point energy (ZPE) constraint model is proposed for classical trajectory simulations of unimolecular decomposition and applied to CH4* → H + CH3 decomposition. With this model trajectories are not allowed to dissociate unless they have ZPE in the CH3 product. If not, they are returned to the CH4* region of phase space and, if necessary, given additional opportunities to dissociate with ZPE. The lifetime for dissociation of an individual trajectory is the time it takes to dissociate with ZPE in CH3, including multiple possible returns to CH4*. With this ZPE constraint the dissociation of CH4* is exponential in time as expected for intrinsic RRKM dynamics and the resulting rate constant is in good agreement with the harmonic quantum value of RRKM theory. In contrast, a model that discards trajectories without ZPE in the reaction products gives a CH4* → H + CH3 rate constant that agrees with the classical and not quantum RRKM value. The rate constant for the purely classical simulation indicates that anharmonicity may be important and the rate constant from the ZPE constrained classical trajectory simulation may not represent the complete anharmonicity of the RRKM quantum dynamics. The ZPE constraint model proposed here is compared with previous models for restricting ZPE flow in intramolecular dynamics, and connecting product and reactant/product quantum energy levels in chemical dynamics simulations.

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Section: Resultsmentioning

confidence: 99%

Zero-Point Energy Constraint for Unimolecular Dissociation Reactions. Giving Trajectories Multiple Chances To Dissociate Correctly

Paul

Hase

2016

J. Phys. Chem. A

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“…A variety of theoretical methods have been advanced for characterizing rovibrational anharmonicity, including diffusion Monte Carlo, , variational approaches, − path integrals, − direct product diagonalization, − empirical methods, , perturbation theories, − specialized methods for torsions, − and other separable approaches. − Many of these methods are designed for spectroscopic applications and are most reliably applied to predict high-accuracy zero point energies and fundamental transition energies. There has been relatively less work done to characterize anharmonicity at high temperatures and energies.…”

Section: Introductionmentioning

confidence: 99%

Anharmonic Rovibrational Partition Functions at High Temperatures: Tests of Reduced-Dimensional Models for Systems with up to Three Fluxional Modes

et al. 2019

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Curvilinear coordinate Monte Carlo phase space integration and a series of full-dimensional fitted potential energy surfaces are used to study the effectiveness of reduced-dimensional models for predicting rovibrational anharmonicity at high temperatures. Fully coupled and fully anharmonic, but classical, rovibrational partition functions Q are computed for 14 species with two or three fluxional modes (inversions or torsions) and as many as 30 degrees of freedom. These results are converted to semiclassical anharmonicity correction factors f and are analyzed alongside results obtained previously for 22 species with up to two fluxional modes. As expected, fluxional species show considerable variation in f at high temperatures; f is as small as 0.2 for acetone and is as large as 9 for methylene glycol at 2500 K. This set of full-dimensional results is used to test the accuracy of reduced-dimensional models where fluxional modes are treated as coupled to one another but as separable from the remaining nonfluxional modes. For most systems, we find that such an approximation is accurate at high temperatures, with average errors in Q of just ∼25%. For some systems, however, larger errors are found, and these are attributed to strong coupling of the fluxional modes to one or more nonfluxional modes. In particular, we identify strong coupling to low-frequency bends for some systems, and we show that by comparing curvilinear and rectilinear harmonic frequencies for the fluxional modes, we can estimate the effect of this coupling on rovibrational anharmonicity. We also quantify the accuracy of the more severe but common assumption of treating fluxional modes as separable from one another, that is, as sets of uncoupled one-dimensional inversions and torsions. This approach can work well for methyl and alkyl rotors, but it is shown to have errors as large as a factor of 7 at high temperatures for more complex systems. Finally, we note that while the present analysis focuses on the treatment of fluxional modes, the collective anharmonicity correction associated with the more numerous nonfluxional modes, although simpler to describe, comprises a significant fraction of the overall anharmonicity.

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“…More approximate and more broadly applicable approaches include empirical corrections for stretches, bends, and torsions; , perturbation theory; ,− specialized treatments for torsions; − and reduced- and one-dimensional anharmonic models . While these approaches can be of great practical utility, the validity of the assumptions and empiricism involved in their specific implementations has not been well studied, particularly for fluxional species at high temperatures and energies, with some exceptions. ,, …”

Section: Introductionmentioning

confidence: 99%

Anharmonic Rovibrational Partition Functions for Fluxional Species at High Temperatures via Monte Carlo Phase Space Integrals

et al. 2018

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Monte Carlo phase space integration (MCPSI) is used to compute full dimensional and fully anharmonic, but classical, rovibrational partition functions for 22 small- and medium-sized molecules and radicals. Several of the species considered here feature multiple minima and low-frequency nonlocal motions, and efficiently sampling these systems is facilitated using curvilinear (stretch, bend, and torsion) coordinates. The curvilinear coordinate MCPSI method is demonstrated to be applicable to the treatment of fluxional species with complex rovibrational structures and as many as 21 fully coupled rovibrational degrees of freedom. Trends in the computed anharmonicity corrections are discussed. For many systems, rovibrational anharmonicities at elevated temperatures are shown to vary consistently with the number of degrees of freedom and with temperature once rovibrational coupling and torsional anharmonicity are accounted for. Larger corrections are found for systems with complex vibrational structures, such as systems with multiple large-amplitude modes and/or multiple minima.

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Approximate calculation of anharmonic densities of vibrational states for very large molecules

Cited by 14 publications

References 68 publications

Zero-Point Energy Constraint for Unimolecular Dissociation Reactions. Giving Trajectories Multiple Chances To Dissociate Correctly

Zero-Point Energy Constraint for Unimolecular Dissociation Reactions. Giving Trajectories Multiple Chances To Dissociate Correctly

Anharmonic Rovibrational Partition Functions at High Temperatures: Tests of Reduced-Dimensional Models for Systems with up to Three Fluxional Modes

Anharmonic Rovibrational Partition Functions for Fluxional Species at High Temperatures via Monte Carlo Phase Space Integrals

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