This Feature Article presents an overview of the current status of ring polymer molecular dynamics (RPMD) rate theory. We first analyze the RPMD approach and its connection to quantum transition-state theory. We then focus on its practical applications to prototypical chemical reactions in the gas phase, which demonstrate how accurate and reliable RPMD is for calculating thermal chemical reaction rate coefficients in multifarious cases. This review serves as an important checkpoint in RPMD rate theory development, which shows that RPMD is shifting from being just one of recent novel ideas to a well-established and validated alternative to conventional techniques for calculating thermal chemical rate coefficients. We also hope it will motivate further applications of RPMD to various chemical reactions.

Cold collisions of light molecules are often dominated by a single partial wave resonance. For the rotational quenching of HD(v = 1, j = 2) by collisions with ground state para-H2, the process is dominated by a single L = 2 partial wave resonance centered around 0.1 K. Here, we show that this resonance can be switched on or off simply by appropriate alignment of the HD rotational angular momentum relative to the initial velocity vector, thereby enabling complete control of the collision outcome.arXiv:1905.04765v1 [quant-ph]

The hydrogen exchange reaction in its HϩD 2 (vϭ0,jϭ0)→HD(vЈϭ0,jЈ)ϩD isotopic variant has been investigated theoretically and experimentally at the collision energies 0.52 eV, 0.531 eV and 0.54 eV. A detailed comparison of converged quantum mechanical scattering calculations and state-to-state molecular beam experiments has allowed a direct assessment of the quality of the different ab initio potential energy surfaces used in the calculations, and strongly favors the newly refined version of the Boothroyd-Keogh-Martin-Peterson surface. The differences found in the calculations are traced back to slight differences in the topology of the potential energy surfaces.

Stereodynamic descriptions of molecular collisions concern the angular correlations that exist between vector properties of the motion of the participating species, including their velocities and rotational angular momenta. Measurements of vector correlations provide a unique view of the forces acting during collisions, and are a stringent test of electronic-structure calculations of molecular interactions. Here, we present direct measurement of the four-vector correlation between initial and final relative velocities and rotational angular momenta in a molecular collision. This property, which quantifies the extent to which a molecule retains a memory of its initial sense of rotation, or handedness, as a function of scattering angle, yields insight into the dynamics of a molecular collision. We report non-intuitive changes in the handedness for specific states and scattering angles, reproduced by classical and quantum scattering calculations. Comparison to calculations on different ab initio potential energy surfaces demonstrates this measurement's exquisite sensitivity to the underlying intermolecular forces.

Zeeman quantum beat spectroscopy has been used to determine the thermal (300 K) rate constants for electronic quenching, rotational energy transfer, and collisional depolarization of OH(AΣ) by H. Cross sections for both the collisional disorientation and collisional disalignment of the angular momentum in the OH(AΣ) radical are reported. The experimental results for OH(AΣ) + H are compared to previous work on the OH(AΣ) + He and Ar systems. Further comparisons are also made to the OH(AΣ) + Kr system, which has been shown to display significant non-adiabatic dynamics. The OH(AΣ) + H experimental data reveal that collisions that survive the electronic quenching process are highly depolarizing, reflecting the deep potential energy wells that exist on the excited electronic state surface.

The inelastic scattering of NO(XΠ) by O(XΣ) was studied at a mean collision energy of 550 cm using velocity-map ion imaging. The initial quantum state of the NO(XΠ, v = 0, j = 0.5, Ω=0.5, 𝜖 = -1, f) molecule was selected using a hexapole electric field, and specific Λ-doublet levels of scattered NO were probed using (1+1) resonantly enhanced multiphoton ionization. A modified "onion-peeling" algorithm was employed to extract angular scattering information from the series of "pancaked," nested Newton spheres arising as a consequence of the rotational excitation of the molecular oxygen collision partner. The extracted differential cross sections for NO(X) f→f and f→e Λ-doublet resolved, spin-orbit conserving transitions, partially resolved in the oxygen co-product rotational quantum state, are reported, along with O fragment pair-correlated rotational state population. The inelastic scattering of NO with O is shown to share many similarities with the scattering of NO(X) with the rare gases. However, subtle differences in the angular distributions between the two collision partners are observed.

The product state-resolved stereodynamics of the reaction of O('D,) with H, have been studied at 300 K at a mean collision energy of ca. 12 kJ mol-I, using polarized, Doppler-resolved laser-induced fluorescence to probe the scattered products, OH(X211i; v' = 0, N', f), and polarized photodissociation of N,O to provide the reagent O('D,) atoms. Product state-resolved differential cross sections, rotational polarizations, and excitation functions are in very good qualitative agreement with the results of quasi-classical trajectory (QCT) calculations, conducted on the Schinke-Lester, SLl ab initio potential energy surface (PES) for the ground electronic state of the collision complex. The experimental and computational results are compared with those obtained in a complementary study of the reaction of O(lD,) with CH, and remarkable parallels have been exposed. The linear and angular momentum vector correlations are all consistent with an "insertion" mechanism, proceeding over an attractive PES, which presents no entrance barrier.

Fully Λ-doublet resolved differential cross sections and collision-induced rotational alignment moments have been measured for the NO(X)-Xe collision system at a collision energy of 519 cm −1 . The experiments combine initial quantum state selection, employing a hexapole inhomogeneous electric field, with quantum state resolved detection, using (1+1 ′ ) resonantly enhanced multiphoton ionization and velocity map ion imaging. The differential cross sections and polarization dependent differential cross sections are shown to agree well with quantum mechanical scattering calculations performed on ab initio potential energy surfaces [J. K los et al. J. Chem. Phys. 137, 014312 (2012)]. By comparison with quasi-classical trajectory calculations, quantum mechanical scattering calculations on a hard-shell potential, and kinematic apse model calculations, the effects of the attractive part of the potential on the measured differential cross sections and collision-induced rotational alignment moments are assessed.