Effect of exchange-correlation functionals and symmetry constraints of electronic structure on the trajectories of reactive molecular collisions

Two computationally efficient implementations of first-principles molecular dynamics are examined using the reactive scattering of two linear ozone molecules to yield three molecules of oxygen. These methods are based on Kohn-Sham density-functional theory in which the charge density is fitted variationally. The linear combination of Gaussian-type orbitals approach is used to construct the orbital and local-potential basis sets. The first method is a completely general approach requiring variational fitting of the exchange-correlation energy density on a grid of points. The second method avoids the grid of points, but is restricted to the $X\alpha$ functional. Key elements of the dynamics calculation include self-consistent-field convergence at each time step, use of fractional occupation numbers to model the configurational mixing that occurs upon bond formation or dissociation, and accurate analytic evaluation of the appropriate forces acting upon the nuclei. Trajectories computed using modern local-density and gradient-corrected functionals, as well as $X\alpha$ $(\alpha$ = 0.6667) treated analytically, are compared. It is found that reactive trajectories obtained using analytic $X\alpha$ and the generalized gradient approximation are very similar when the initial kinetic energy is well above threshold for reaction. Under similar conditions, trajectories resulting from use of modern local-density functionals exhibit somewhat distinctive behavior. These results suggest that the particularly efficient analytic $X\alpha$ approach is suitable for preliminary investigations of chemical reactions using first-principles molecular dynamics based on density-functional theory.