Hello,
What I’d do is to run the PWE bandsolver and then fit the curves I’m interested in, so I can relate ‘w’ with ‘k’. Then I’d derivate tihs expression (dw/dk = vg) and I’d obtain vg, which is the group velocity and it gives us the neff.
Sorry about this quick explanation. I hope it is helpfull, I’m in a hurry :).
Regards,
DCM