Spectral_function
The spectral function for the normalized state \psi is defined as (sometimes modulo a factor 2*pi):
where H is the Hamiltonian and the overline denotes averaging over disorder configurations. By construction, it is normalized to unity:
and its average value is:
The spectral function can be computed by Fourier transform of the autocorrelation function:
and:
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\psi(t) is computed using the Temporal propagation method. For the standard spectral function, \psi(0) is a plane wave, but the program allows any initial state.
One has to specify the required energy resolution (which will give the total propagation time 2\pi/energy_resolution) and the total energy range: because of the performed FFT, the spectrum is folded in an interval Delta E = 2\pi/dt, where dt is the elementary time step for propagation. Thus, the program uses dt=2\pi/energy_range. The spectrum is output in the energy range chosen by the user.
Note that the CPU time spent in the Fourier transform time->energy is usually negligible.
The spectral function can be computed in the interacting case, simply as the Fourier transform of the nonlinear C(t), but its physical meaning is not completely clear.