Limitations of ab initio molecular dynamics simulations of simple reactions: F + H₂ as a prototype

We present the first use of ab initio molecular dynamics (AIMD) with correlated wavefunctions to study a gas phase reaction. Our benchmark system is the well-studied F + H₂ reaction, The neutral, unstable species FH₂ is created by removing an electron from an equilibrated anion (FH₂)^-, simulating the photoelectron spectroscopy experiments of Neumark and co-workers. These experiments attempt to probe the transition state (TS) region by creating a negative ion that may have a structure similar to the neutral reaction TS. We then follow the dynamics of the neutral system on its Born-Oppenheimer surface, both forward and backward in time, to see how close the anion equilibrium geometry truly is to the neutral TS for the F + H₂ → HF + H reaction. We then perform AIMD from the actual neutral TS for comparison. All electronic energy and gradient calculations are at the complete active space self-consistent field level. We find for this system that our level of approximation leads only to a qualitatively correct description. It allows us, however, to show that at finite temperature the anion geometry is markedly different from the neutral TS structure in that instantaneous electron detachment forms an FH₂ complex lying entirely in the reactants valley. We propose that analysis of the asymptotic fragments in the photodetachment experiment will almost invariably detect F + H₂ and very rarely FH + H, and we also discuss which conditions would lead to a different conclusion.