Theoretical analysis of the nonlinear susceptibility of hydrogen atom*

The nonlinear refractive index of air and other gases plays an important role in the understanding of the propagation of intense laser pulses in the atmosphere. Using a numerical basis state method $[1]$ we have performed accurate calculations of the nonlinear susceptibility in hydrogen atom. Currently, models of filamentation mainly use perturbative approaches to determine the nonlinear susceptibility. We compare our results from ab-initio calculations of the nonlinear susceptibity with those from a perturbative calculation. The comparison allows us to determine an intensity at which perturbation theory breaks down for these kinds of calculations. \\[0pt] [1] S.H. Chen et al., Phys. Rev. A {\bf 86}, 013410 (2012)