Molecular Dynamics in Strong Laser Fields: A New Algorithm for ab Initio Classical Trajectories.

A new, more accurate Hessian-based predictor-corrector algorithm has been developed for simulating classical trajectories of molecules in intense laser fields. The first and second derivatives of the gradient with respect to the electric field are included in the both the predictor and the corrector steps for integrating trajectories. A Taylor expansion of the gradient is used as the surface for integrating the predictor step; a distance weighted interpolant of the gradient is employed for the corrector step. Test trajectories were calculated for HCO(+) in a continuous 10 μm, 2.9 × 10(14) W cm(-2) laser field and triplet allene dication in a 10 μm, 5.7 × 10(13) W cm(-2) four cycle cosine pulse. The first derivative of the gradient with respect to the electric field makes a significant contribution, while the second derivative makes a smaller contribution and can be neglected. To reduce the cost, the Hessian can be updated for several steps before being recalculated. The calculations indicate that a step size of Δt = 0.25 fs and 20 updates is efficient and reliable for these test systems.