Accurate vibrational-rotational partition functions and standard-state free energy values for H2O2 from Monte Carlo path-integral calculations

Abstract

Accurate quantum mechanical partition functions and absolute free energies of H(2)O(2) are determined using a realistic potential energy surface [J. Koput, S. Carter, and N. C. Handy, J. Phys. Chem. A 102, 6325 (1998)] for temperatures ranging from 300 to 2,400 K by using Monte Carlo path integral calculations with new, efficient polyatomic importance sampling methods. The path centroids are sampled in Jacobi coordinates via a set of independent ziggurat schemes. The calculations employed enhanced-same-path extrapolation of trapezoidal Trotter Fourier path integrals, and the paths were constructed using fast Fourier sine transforms. Importance sampling was also used in Fourier coefficient space, and adaptively optimized stratified sampling was used in configuration space. The free energy values obtained from the path-integral calculations are compared to separable-mode approximations, to the Pitzer-Gwinn approximation, and to values in thermodynamic tables. Our calculations support the recently proposed revisions to the JANAF tables.

Exploring the effect of anharmonicity of molecular vibrations on thermodynamic properties

“…Both are in very good agreement with the 1.008 value predicted by Truhlar et al This is to be expected at room temperature, since, as noted by Truhlar et al, the rigid-rotor-harmonic-oscillator approximation is already very close to the accurate rotovibrational partition function due to the lack of floppy modes in CH 4 . For H 2 O 2 , the calculated ZPE͑0͒ ELC = 16.46 kcal/ mol is in very good agreement with the value of ϳ16.37 kcal/ mol estimated by Lynch et al 30 Finally, the calculated room temperature standard-state Gibbs free energy of 13.64 kcal/ mol underestimates by less than 0.5 kcal/ mol the more accurate values obtained by Lynch et al 29 and Dorofeeva et al…”

Quantum mechanical single molecule partition function from path integral Monte Carlo simulations

“…This system is anharmonic in the O-O bond rotation and absolute partition functions have been reported in the literature. 4 The PES for hydrogen peroxide is well characterized and is available in the form of a down-loadable subroutine. 7,20 Lynch et al 4 used this potential energy surface and calculated the rotational-vibrational partition function of H 2 O 2 at 300K to be 3.00 × 10 −9 .…”

“…4 The PES for hydrogen peroxide is well characterized and is available in the form of a down-loadable subroutine. 7,20 Lynch et al 4 used this potential energy surface and calculated the rotational-vibrational partition function of H 2 O 2 at 300K to be 3.00 × 10 −9 . Using the same potential to define V 0 , our PIMC calculations gave a value of 2.95±0.03 × 10 −9 for the partition function.…”

“…We used 100000 iterations per block in these simulations. Since the simulation contains 100 blocks the total number of MC iterations is 10 7 which is roughly of the same order as reported by Lynch et al 4 However we have used Cartesian coordinates for sampling, which makes our methodology more general and applicable to more complicated systems given below. To define the harmonic reference potential V 1 , we used the simple forcefield of the form given in Eq.…”

“…Previous path integral Monte Carlo calculations by Lynch et al 4 have used a free-particles reference state which makes the perturbation between real and reference system quite large. We have used a harmonic potential as the reference state, which makes the real and reference systems quite similar.…”