We are trying to fit a potential for HfO2 with partial charges for Hf and O. We would like to represent O using the shell model of Dick and Overhauser. I understood that I can do this using monopoleq tag under observables. For example
1 2.400 #Hf
2 0.4241 #O core
3 0.4241 # O core
4 -1.6241 # O shel
5 -1.6241 # Oshel
The two issues I have are:
(1) how to tell GULP that these charge are initial guesses rather than observables to fit for.
(2) Somehow GULP ends up with different charges for the two oxygens. Is it possible to enforce that both oxygen cores should have the same charge and both oxygen shels shouls have the same charge? From symmetry (nonoclinc space group 14) the two oxygens are different but I still would like to enforce the same charges on both of them.
Any help is highly appreciated.