  We present an approximation for efficient calculation of the Lindhard
susceptibility $\chi^{L}(q,\omega)$ in a periodic system through the use of
simple products of real space functions and the fast Fourier transform (FFT).
The method is illustrated by providing $\chi^{L}(q,\omega)$ results for the
electron doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ extended over several
Brillouin zones. These results are relevant for interpreting inelastic X-ray
scattering spectra from cuprates.
