These two fields are plotted in Fig. for the same parameters A and ω 0 . The field is turned on at t = 0 where the MOs {ψ k (r, 0)} represent the ground-state Kohn-Sham solution. Following sufficient time propagation, the HHG spectrum is computed from the Fourier transform ( F) of the time-dependent dipole acceleration, d A (t). The dipole acceleration is the second derivative of the dipole moment, µ(t): (20).