A Time-Dependent Born-Oppenheimer Approximation with Exponentially Small Error Estimates

We present the construction of an exponentially accurate time-dependent Born-Oppenheimer approximation for molecular quantum mechanics. We study molecular systems whose electron masses are held fixed and whose nuclear masses are proportional to ${ϵ}^{-4}$, where $ϵ$ is a small expansion parameter. By optimal truncation of an asymptotic expansion, we construct approximate solutions to the time-dependent Schr"odinger equation that agree with exact normalized solutions up to errors whose norms are bounded by $\ds C \exp(-\gamma/\epsilon^2)$,forsomeCand$\gamma>0$.