Dear Honghu,
I understand you get the error message above when you leave WHAT(3) blank in EMFCUT with SDUM=PROD-CUT. Let us know if this is not the case.
The WHAT(3) above is a dimensionless number controlling the fraction of atomic electrons effectively contributing to multiple Coulomb scattering.
As stated in the manual, WHAT(3)=1.0 (all atomic electrons contributing to multiple Coulomb scattering) is appropriate for delta-ray production thresholds of ~100 keV or larger.
Instead, in your case the delta-ray production threshold is 10 keV, so as suggested in the manual you should put a low value of WHAT(3), say 1e-5, otherwise you’d be effectively double counting the contribution of collisions with atomic electrons to the deflection: once via explicit delta-ray production, and again in multiple Coulomb scattering.
There’s no one-size-fits-all rule, but what is suggested in the manual is fair enough: one should linearly interpolate from WHAT(3)=1e-5 (for delta-ray production thresholds in the order a few keV like in your case) to 1.0 (for delta-ray production 100 keV or larger).
Cheers,
Cesc