Tunneling Splittings in Water Clusters from Path Integral Molecular Dynamics.

We present calculations of tunneling splittings in selected small water clusters, based on a recently developed path integral molecular dynamics (PIMD) method. The ground-rotational-state tunneling motions associated with the largest splittings in the water dimer, trimer, and hexamer are considered, and we show that the PIMD predictions are in very good agreement with benchmark quantum and experimental results. As the tunneling spectra are highly sensitive to both the details of the quantum dynamics and the potential energy surface, our calculations are a validation of the MB-Pol surface as well as the accuracy of PIMD. The favorable scaling of PIMD with system size paves the way for calculations of tunneling splittings in large, nonrigid molecular systems with motions that cannot be treated accurately by other methods, such as the semiclassical instanton

Quantum scattering calculations on chemical reactions

This review discusses recent quantum scattering calculations on bimolecular chemical reactions in the gas phase. This theory provides detailed and accurate predictions on the dynamics and kinetics of reactions containing three atoms. In addition, the method can now be applied to reactions involving polyatomic molecules. Results obtained with both time-independent and time-dependent quantum dynamical methods are described. The review emphasises the recent development in time-dependent wave packet theories and the applications of reduced dimensionality approaches for treating polyatomic reactions. Calculations on over 40 different reactions are described

An ab lnitio Calculation of the Low Rotation-Vibration Energies of the CO Dimer

On the basis of a previously published ab initio potential surface for the CO dimer [A. van der Pol, A. van der Avoird and P. E. S. Wormer, J. Chem. Phys.92, 7498-7504 (1990); the potential assumes fixed CO bondlengths] we have calculated the J < 3 rotation-vibration energies below 25 cm-1. This should be suitable for interpreting the absorption spectrum of CO dimer cooled to about 10 K. We have made uncoupled one- and three-dimensional calculations of the energies, as well as two different four-dimensional calculations. The first four-dimensional calculation uses an adiabatic separation of the intermolecular coordinate R, and the second uses the full close-coupling method. It is shown that because of the strong couplings on this surface there are differences between the results obtained from these calculations. However, even using the close-coupling results we cannot assign the five published experimental frequencies of CO dimer at 10K [P. A. Vanden Bout, J. M. Steed, L. S. Bernstein, and W. Klemperer, Astrophys. J.234, 503-505 (1979)]. © 1993 Academic Press. All rights reserved