CHARMM charge parameterization question!!



Dear CCLs:
 I have an abnormal residue in the simulating protein.
  I want ask you how to calculation this residue charge for further
 simulation(CHARMM 19 FF).
 I use the G98 to calculate the ESP charge, the N terminus and C terminus block
 with CH3CO and NHCH3 to mimic the protein chain environment. But I found when I
 remove the block group, the total charge of this abnormal residue is not zero.
 How can I do ?
 any reference, articles and suggestion will be helpful?
 Thanks!!!
 Best regards!
             bxiong "-at-" mail.shcnc.ac.cn
 Xiong Bin
 Shanghai Institute of Materia Medica, C.A.S.
 phone:021-64311833-222(office)