The key
mkes the code switching from the berry phase formulation for the polarization and the coupling with the external field, to a formulation based on the dipoles.
The latter is in principle correct only to linear order in the external field. So, if you are looking into some non linear effect, you might get some error in your results.
Remember that any pump and probe signal is a non linear process in the sum of pump plus probe.
However, it is not easy to predict a priory how big such error would be. In some cases it is negligible, in others it is important.
To give you an example, in this work
https://journals.aps.org/prb/abstract/1 ... 107.205203 I could not get any signal in the transient spectrum between 0 and 3 eV (e.g. in the region of exciton - exciton transitions) by using the coupling with the fields based on the dipoles. On the other hand, in this work
https://journals.aps.org/prmaterials/ab ... s.5.083803 I could hardly see any difference in the transient signal. In the two works I'm doing simulations on the same material, just looking into the transient signal at very (!) different energy ranges.
Hope it helps.
Best,
D.