Modelling intensity volume averaging in ab initio calculations of High Harmonic Generation

We present an approach to assess the survival of single-atom effects in the macroscopic high-harmonic generation (HHG) spectrum, by accounting for focal volume averaging. We apply this technique to R-matrix with time-dependence (RMT) studies, which are designed to include the full multielectron response of an atom. Such an approach allows the assessment of which features of an experimentally-measured HHG spectrum of diffuse gases may be traced directly to single-atom effects and vice-versa. While accounting for the phase of harmonics produced at different locations in the focal volume gives the most accurate results, a simplified approach, using a smaller number of RMT calculations, is found to provide comparable conclusions. We apply these approaches to compute intensity-averaged harmonic spectra in two different experimental regimes.