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Anharmonic state counts and partition functions for molecules via classical phase space integrals in curvilinear coordinates
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10.1063/1.4804420
/content/aip/journal/jcp/138/19/10.1063/1.4804420
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/19/10.1063/1.4804420

Figures

Image of FIG. 1.
FIG. 1.

Comparison of the anharmonicity correction to ρ for CH computed using Cartesian normal-mode and z-matrix coordinates.

Image of FIG. 2.
FIG. 2.

(a) Relative standard deviation and (b) relative maximum error for at three energies: 10 000 cm, 15 000 cm, and 20 000 cm using both coordinate systems. Each point on the plot corresponds to the convergence metrics evaluated for one of the three energies using the specified coordinate system and for some value of . The solid curves are the best fits of the data points to the form · .

Image of FIG. 3.
FIG. 3.

(a) One-dimensional state density for the HC–C–H vibration for two choices of the bending coordinate and the harmonic approximation to it. The energy of the barrier between the two equivalent minima is shown as the pink vertical line. (b) The potential energy curves along two coordinates.

Image of FIG. 4.
FIG. 4.

(a) Relative standard deviation and (b) relative maximum error for at two energies, 5000 cm and 10 000 cm, using both coordinate systems. The solid curves are the best fits of the data points to the form · .

Image of FIG. 5.
FIG. 5.

Anharmonicity corrections to the (a) density of states and (b) partition function for the vinyl radical.

Tables

Generic image for table
Table I.

Comparison of vibrational partition functions obtained using the harmonic approximation, the Pitzer-Gwinn corrected harmonic partition function, and VCI. Results above 1500 K are not shown for VCI because it became difficult to converge sufficient vibrational states at high enough energies to converge .

Generic image for table
Table II.

Comparison of partition functions calculated with and without the inclusion of coupling between vibrations and rotations. All partition functions are calculated classically and numbers in square brackets correspond to powers of ten. The columns, respectively, are: the purely vibrational harmonic oscillator partition function, ; the purely vibrational anharmonic sampled partition function, ; the rovibrational partition function using the harmonic vibrations and the rigid rotor approximation, ; the rovibrational partition function using the anharmonic sampled vibrations and the rigid rotor approximation, ; and the rovibrational partition function computed fully using the sampling algorithm, . The anharmonicity corrections are defined as = / , = / , and = / .

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/content/aip/journal/jcp/138/19/10.1063/1.4804420
2013-05-21
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Anharmonic state counts and partition functions for molecules via classical phase space integrals in curvilinear coordinates
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/19/10.1063/1.4804420
10.1063/1.4804420
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