The treatment of classically forbidden electronic transitions in semiclassical trajectory surface hopping calculations

Citation
Aw. Jasper et al., The treatment of classically forbidden electronic transitions in semiclassical trajectory surface hopping calculations, J CHEM PHYS, 115(4), 2001, pp. 1804-1816
Citations number
57
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF CHEMICAL PHYSICS
ISSN journal
00219606 → ACNP
Volume
115
Issue
4
Year of publication
2001
Pages
1804 - 1816
Database
ISI
SICI code
0021-9606(20010722)115:4<1804:TTOCFE>2.0.ZU;2-0
Abstract
A family of four weakly coupled electronically nonadiabatic bimolecular mod el photochemical systems is presented. Fully converged quantum mechanical c alculations with up to 25 269 basis functions were performed for full-dimen sional atom-diatom collisions to determine the accurate scattering dynamics for each of the four systems. The quantum mechanical probabilities for ele ctronically nonadiabatic reaction and for nonreactive electronic deexcitati on vary from 10(-1) to 10(-5). Tully's fewest-switches (TFS) semiclassical trajectory surface-hopping method (also called molecular dynamics with quan tum transitions or MDQT) is tested against the accurate quantal results. Th e nonadiabatic reaction and nonreactive deexcitation events are found to be highly classically forbidden for these systems, which were specifically de signed to model classically forbidden electronic transitions (also called f rustrated hops). The TFS method is shown to systematically overestimate the nonadiabatic transition probabilities due to the high occurrence of frustr ated hops. In order to better understand this problem and learn how to best minimize the errors, we test several variants of the TFS method on the fou r new weakly coupled systems and also on a set of three more strongly coupl ed model systems that have been presented previously. The methods tested he re differ from one another in their treatment of the classical trajectory d uring and after a frustrated hopping event. During the hopping event we fin d that using a rotated hopping vector results in the best agreement of semi classical and quantal results for the nonadiabatic transition probabilities . After the hopping event, we find that ignoring frustrated hops instead of reversing the momentum along the nonadiabatic coupling vector results in t he best agreement with the accurate quantum results for the final vibration al and rotational moments. We also test the use of symmetrized probabilitie s in the equations for the TFS hopping probabilities. These methods systema tically lead to increased error for systems with weakly coupled electronic states unless the hopping probabilities are symmetrized according to the el ectronic state populations. (C) 2001 American Institute of Physics.