  We introduce a new computational approach for femtosecond pulse propagation
in the transparency region of gases that permits full resolution in three space
dimensions plus time while fully incorporating quantum coherent effects such as
high-harmonic generation and strong-field ionization in a holistic fashion.
This is achieved by utilizing a one-dimensional model atom with a
delta-function potential which allows for a closed-form solution for the
nonlinear optical response due to ground-state to continuum transitions. It
side-steps evaluation of the wave function, and offers more than one
hundred-fold reduction in computation time in comparison to direct solution of
the atomic Schr\"odinger equation. To illustrate the capability of our new
computational approach, we apply it to the example of near-threshold harmonic
generation in Xenon, and we also present a qualitative comparison between our
model and results from an in-house experiment on extreme ultraviolet generation
in a femtosecond enhancement cavity.
