[gmx-users] charge group number

[Chetan Mahajan]

Hi All

I am just learning how to assign charge group number. For TiO2 slab,
 amber2gromacs tool (acpype) I used assigns cgnr increasing consecutively
from Ti atoms followed by oxygen atoms. Clearly charge group here ( either
Ti or O) does not have zero charge. Can anyone throw light on why cgnr
should be like this (individual atoms) here?

Also in formate anion, should I have charge groups (charge in the bracket)
as
follows:

charge group 1 : C (0.9) and O (-0.88) together
charge group 2 : O (-0.88)
charge group 3: H (-0.147)

Thanks in advance,

regards
Chetan