triqs_ctseg::measure_nnt
#include <triqs_ctseg.hpp>
class measure_nnt
measurement for the density-density correlation function in imaginary time
Measures of the imaginary-time density-density correlation function
\[\chi^{\sigma\sigma'}_{ab}(\tau) = \langle T_\tau
n_{a\sigma}(\tau)n_{b\sigma'}(0) \rangle\]
It is triggered through the switch
measure_nnt.
- In contrast to the Green’s function, the measurement slows down considerably
with the number of points
n_tau_nnon the grid.- In most cases, it is more efficicient to measure it on Matsubara frequencies
as specified through
measure_nnw:
\[\chi^{\sigma\sigma'}_{ab}(i\nu_m) = \int_0^\beta d\tau\, e^{i\nu_m\tau}
\chi^{\sigma\sigma'}_{ab}(\tau)\]
where the number of bosonic frequencies \(\nu_m=2m\pi/\beta\) is specified through
n_w_b_nn.
Note
Does not exploit time translational invariance