where F = F0 +E(t)•r. Our implementation of the CAP is similar to that used by Schlegel and coworkers to study strong-field ionization dynamics, and which we have previously used to compute broadband x-ray absorption spectra. The real-space CAP function W CAP (r) is constructed from a set of overlapping, atom-centered spherical potentials, each of which is zero within a cutoff radius r 0 then rises quadratically with curvature η. Explicitly, these functions are