Efficient Calculation of Statistical and Dynamical Reaction Rates for Large Dimensional Molecular Systems

Principal Investigator

Harold Schranz

Research School of Chemistry

Knowledge of how quickly chemical reactions occur is an essential ingredient in the rigorous modelling
of combustion, industrial and atmospheric reaction systems. This project focuses on the development of new methods for the accurate prediction of rate constants for chemical reaction. Crucial to this development is a more complete understanding of how and on what timescale energy moves about a molecule (IVR) and between molecules (collisional energy transfer). The current focus is on the role of the intermolecular potential energy and quantum effects on collisional energy transfer in highly excited molecules.



Terry Frankcombe

Research School of Chemistry and Department of Chemistry




s10 - VPP, PC



What are the results to date and the future of the work?

A recent series of computational studies of the intermolecular potential energy surface and collisional energy transfer of the CO2 + Ar system was performed in collaboration with Terry Frankcombe and Dr. Rob Stranger. High end ab initio and DFT techniques were employed with sophisticated fitting schemes to yield a variety of different models of the intermolecular potential surface.

The first figure shows a contour plot (in cm-1) of the intermolecular potential surface of CO2-Ar calculated at the MP4/6-311+G(2d) level as a function of C-Ar distance and angle from the linear approach.

Simulations run on these surfaces indicated that a significant role in the efficiency of collisional energy transfer is played by the repulsive wall: the average steepness correlated well with the magnitude of energy transfer.

The second figure displays average energy transfer as a function of bath temperature. Initial internal energy 30 kcal/mol. Noble gas Lennard-Jones (circles), ADF Lennard-Jones (plusses), MP4 Lennard-Jones (crosses), Morse (squares) and MP2 Lennard-Jones (diamonds) potentials.

One of the future aims of this project is to develop a useful method to incorporate quantum effects in classical simulations. This component may involve a binational collaboration with Prof. John Barker of the University of Michigan who has proposed a simple model for incorporating quantum effects in the transition probability P(E',E) governing energy transfer. To create the input for this statistical model we intend to perform classical trajectory simulations

- Appendix A


and calculate quantum corrections by semiclassical methods. A comparison with the simpler but ad hoc constraint procedures will also be of interest in exploring the classical-quantum correspondence. A further related component is a proposed study of collisional energy transfer from the dilute to dense fluid phase. How energy transfer and chemical reactions are affected by the transition from the dilute gas to liquid phase regime is of much current experimental interest.

What computational techniques are used?

Quantum chemical ab initio packages (GAUSSIAN) and density functional theory packages (ADF) were used for calculating points on the intermolecular potential. Given a model of the colliding system (e.g. an excited target molecule and a thermal projectile molecule), described by a global potential energy surface including intra- and intermolecular parts, the classical equations of motion were solved for a specified ensemble of initial conditions before the collision and thereby ending up with a set of final states after the collision is over. Home grown efficient vectorised trajectory codes were employed for this portion of the project and production runs were performed using the VPP and Power Challenge supercomputers. A variety of detailed and averaged quantities were extracted from such studies.

Appendix A -



T. J. Frankcombe, R. Stranger, H. W. Schranz, The intermolecular potential energy surface of CO2-Ar and its effect on collisional energy transfer, Internet Journal of Chemistry 1 (1998) 12.

- Appendix A