Overtone spectrum of stannane: experiment and theoretical analysis

171

215

New analytical model for the ozone electronic ground state potential surface and accurate ab initio vibrational predictions at high energy rangeWe present an ab initio potential energy surface for the ground electronic state of ozone. It is global, i.e., it covers the three identical C 2v ͑open͒ minima, the D 3h ͑ring͒ minimum, as well as the O( 3 P)ϩO 2 ( 3 ⌺ g Ϫ ) dissociation threshold. The electronic structure calculations are performed at the multireference configuration interaction level with complete active space self-consistent-field reference functions and correlation consistent polarized quadruple zeta atomic basis functions. Two of the O-O bond distances, R 1 and R 2 , and the O-O-O bending angle are varied on a regular grid ͑ca. 5000 points with R 1 уR 2 ). An analytical representation is obtained by a three-dimensional cubic spline. The calculated potential energy surface has a tiny dissociation barrier and a shallow van der Waals minimum in the exit channel. The ring minimum is separated from the three open minima by a high potential barrier and therefore presumably does not influence the low-temperature kinetics. The dissociation energy is reproduced up to 90% of the experimental value. All bound states of nonrotating ozone up to more than 99% of the dissociation energy are calculated using the filter diagonalization technique and employing Jacobi coordinates. The three lowest transition energies for 16 O 3 are 1101.9 cm Ϫ1 ͑1103.14 cm Ϫ1 ͒, 698.5 cm Ϫ1 ͑700.93 cm Ϫ1 ͒, and 1043.9 cm Ϫ1 ͑1042.14 cm Ϫ1 ͒ for the symmetric stretch, the bending, and the antisymmetric stretch modes, respectively; the numbers in parentheses are the experimental values. The root-mean-square error for all measured transition energies for 16 O 3 is only 5 cm Ϫ1 . The comparison is equally favorable for all other isotopomers, for which experimental frequencies are available. The assignment is made in terms of normal modes, despite the observation that with increasing energy an increasing number of states acquires local-mode character. At energies close to the threshold a large fraction of states is still unambiguously assignable, particularly those of the overtone progressions. This is in accord with the existence of stable classical periodic orbits up to very high energies.

Section: B Polyad Structure and Local-mode Statesmentioning

confidence: 99%

The vibrational energies of ozone up to the dissociation threshold: Dynamics calculations on an accurate potential energy surface

Siebert¹,

Fleurat‐Lessard²,

Schinke³

et al. 2002

171

215

“…Water has been widely studied as a prototype for vibrations in a polyatomic molecule. 20 For our purposes the best set of partition functions obtained by eigenvalue summation for a given potential energy surface are those of Partridge and Schwenke. 21 Therefore we have chosen to use their potential surface and compare to their data.…”

Section: Calculations and Discussionmentioning

confidence: 99%

“…Tables IV and V give the Q corr,K,KЈ (T) calculations together with two different extrapolations to the Kϭϱ limit. 20 Our partition function calculations contain errors from several sources. Statistical error is likely to be the largest source of error.…”

Section: Calculations and Discussionmentioning

confidence: 99%

Extrapolation and perturbation schemes for accelerating the convergence of quantum mechanical free energy calculations via the Fourier path-integral Monte Carlo method

Mielke

Srinivasan

Truhlar

2000

A new Fourier path integral method, a more general scheme for extrapolation, and comparison of eight path integral methods for the quantum mechanical calculation of free energies We present two simple but effective techniques designed to improve the rate of convergence of the Fourier path-integral Monte Carlo method for quantum partition functions with respect to the Fourier space expansion length, K, especially at low temperatures. The first method treats the high Fourier components as a perturbation, and the second method involves an extrapolation of the partition function ͑or perturbative correction to the partition function͒ with respect to the parameter K. We perform a sequence of calculations at several values of K such that the statistical errors for the set of results are correlated, and this permits extremely accurate extrapolations. We demonstrate the high accuracy and efficiency of these new approaches by computing partition functions for H 2 O from 296 to 4000 K and comparing to the accurate results of Partridge and Schwenke.

“…In this way, simple block diagonal Hamiltonian matrix models are obtained. 7 This approach is similar to the normal mode models although details and physical interpretation of the Hamiltonian coefficients differ.…”

Section: Introductionmentioning

confidence: 99%

Torsional motion and vibrational overtone spectroscopy of methanol

Hänninen

Horn

Halonen

1999

An internal coordinate Hamiltonian model has been constructed to model torsional motion in the OH stretching vibrational overtone region of methanol, CH 3 OH. The model includes harmonic couplings between OH and CH stretching vibrations and Fermi resonance interactions between OH stretches and COH bends and between CH stretches and CH 2 bends. A symmetrized basis set has been used to form block diagonal Hamiltonian matrices with strong resonance couplings off-diagonal. Observed torsional levels of the excited vibrational states have been used as data in a least squares optimization of the model parameters, some of which have been estimated by ab initio calculations. The experimentally observed increase in the effective torsional barrier in moving to highly excited OH stretching states has been explained by the model.