Scanning Reactive Pathways with Orbital Biased Molecular Dynamics.

To accelerate reactive events in molecular dynamics simulations we introduce a general bias potential scheme which depends only on the electronic degrees of freedom of the reactive system. This electronic reaction coordinate, which is expressed in terms of a penalty function of the one-electron orbital energies, has been applied to study different reaction pathways of s-cis-butadiene. Three different reactive channels have been identified:  the cis/trans isomerization, the s-cis/s-trans isomerization, and the symmetry allowed cyclization. For the latter, despite the fact that the Woodward-Hoffmann rules are guided by the butadiene frontier orbitals, biasing only these orbitals is not enough to drive the system toward cyclization, but a low-lying valence shell orbital needs to be included.