Trajectory short-lived

The integration of forces between all molecules over several thousand time-steps produces particle trajectories from which time-averaged macroscopic properties can be computed. In MD the simulation is limited by the computer storage capacity and speed, so that short-lived phenomena (100-1000 ps) are generally calculated. 

There were four short trajectories on the PMl surface and 10 such trajectories on the AMI surface. All of these short-lived direct trajectories proceed with counterclockwise motion about the C -C bond leading exclusively to inversion about C5. No short-lived trajectory gives retention product nor do they reside in the vicinity of the diradical 4 for any appreciable time. One may consider these trajectories as following the Woodward-Hoffman allowed, concerted path, except that it occurs over a PES containing a diradical ... 

With these tools at hand, theoretical chemists were in a position to consider the reactivity problem in a new perspective. It became possible to predict which reaction mechanisms would lead to reasonable or to abnormal energy barriers, and to calculate with acceptable accuracy barrier heights, complete potential surfaces, or even trajectories on these surfaces. The geometry and electronic structure of short-lived species such as activated complexes or unstable reaction intermediates were obtained this way. This gave the experimental scientist invaluable information about species not amenable to experimental investigations. 

This minimally dynamic approach has been applied to both bimolecular and unimolecular reactions a typical result for the latter case is shown in Fig. 6. In this case we consider the dissociation of CCH on two different potential surfaces due to Wolf and Hase.36 These authors classified the first surface (their case IIC) as yielding RRKM dissociation, whereas their surface IIA yielded non-RRKM dynamics. The exact trajectory results for translational, vibrational, and rotational distributions for these two cases are shown as solid histograms in Fig. 6. The minimally dynamic construction, which requires only short-lived trajectory calculations, are shown as dashed histograms in the same figure and are seen to be in excellent agreement with the exact results. 

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