Computes the effect of a gradient pulse on the sample average density matrix using Edwards formalism. It is assumed that the effect of diffusion is negligible, that the gradient is linear, and that it is antisymmetric about the middle of the sample. Syntax:
rho - spin system state vector
L - system Liouvillian
g_amp - gradient amplitude, Gauss/cm
s_len - sample length, cm
g_dur - gradient pulse duration, seconds
s_fac - gradient shape factor, use 1 for square gradient pulses
Note: the function integrates over sample coordinates - subsequent gradient pulses would not refocus the magnetization that it has defocused. To simulate a gradient sandwich, use grad_sandw.m function. More information on the subject is available in Luke's paper (http://dx.doi.org/10.1016/j.jmr.2014.01.011).