New Stress Test for Ring Polymer Molecular Dynamics: Rate Coefficients of the O(³P) + HCl Reaction and Comparison with Quantum Mechanical and Quasiclassical Trajectory Results

Abstract

In the past decade, ring polymer molecular dynamics (RPMD) has emerged as a very efficient method to determine thermal rate coefficients for a great variety of chemical reactions. This work presents the application of this methodology to study the O­(3P) + HCl reaction, which constitutes a stringent test for any dynamical calculation due to rich resonant structure and other dynamical features. The rate coefficients, calculated on the 3A′ and 3A″ potential energy surfaces (PESs) by Ramachandran and Peterson [J. Chem. Phys.20031199590], using RPMD and quasiclassical trajectories (QCT) are compared with the existing experimental and the quantum mechanical (QM) results by Xie et al. [J. Chem. Phys.2005122014301]. The agreement is very good at T > 600 K, although RPMD underestimates rate coefficients by a factor between 4 and 2 in the 200–500 K interval. The origin of these discrepancies lies in the large contribution from tunneling on the 3A″ PES, which is enhanced by resonances due to quasibound states in the van der Waals wells. Although tunneling is fairly well accounted for by RPMD even below the crossover temperature, the effect of resonances, a long-time effect, is not included in the methodology. At the highest temperatures studied in this work, 2000–3300 K, the RPMD rate coefficients are somewhat larger than the QM ones, but this is shown to be due to limitations in the QM calculations and the RPMD are believed to be more reliable.

Capturing quantum effects with quasi-classical trajectories in the D + H+3 → H2D+ + H reaction

“…Previous tests have shown RPMD rate constants can be 10-20% below exact quantum mechanical results. 39,40 The significant disagreement between RPMD and the present ZPE-E and 1GB-NH results below 150 K may be due in part to approximations within the RPMD treatment of this system. Moreover, since rate constants for complex forming reactions, such as the title reaction, are difficult to converge with RPMD, alternative QCT-based methods including quantum effects are desirable.…”

A Machine Learning Approach for Rate Constants. II. Clustering, Training, and Predictions for the O(3P) + HCl → OH + Cl Reaction

“…Looking ahead, the exact corrections to the Eckart transmission coefficient can be done for this reaction for both PESs. There is more data for the reactive 3 A″ PES because the quantum rate constant was reported from 200 to 3000 K. For the higher-barrier 3 A′ PES, useful data are available above roughly 600 K. 29,45 These will be included in a new data set that will be made readily available as described in detail in the next section.…”

“…Numerous tests of them indicate a level of accuracy of about a factor of 1.5–2 for the rate constant at low temperatures. For example, for H + H 2 , OH + H 2 , and H + CH 4 , this accuracy was reported for the RPMD method compared to benchmark results. ,− A very recent “stress test” of RPMD was reported for the O­( 3 P) + HCl reaction (on 3 A′ and 3 A″ potentials) . More discussion of this reaction will be given below.…”

“…We thank Professor Marta Menendez for sending data in ref 29 and also for sending TST rate constants for the 3 A′ and 3 A″ PESs. J.M.B.…”

“…21,26−28 A very recent "stress test" of RPMD was reported for the O( 3 P) + HCl reaction (on 3 A′ and 3 A″ potentials). 29 More discussion of this reaction will be given below.…”