parameters of amber force field for Hydroxide ion?



Hello, 
where can I find the emperical force field paramters for the hydroxide ion?
I am modelling an enzyme system, containing Zinc ions with water/hydroxide ligands around in the active site. For such kind of system, is there anyone having the experience of treating it by pure  molecular mechanic method?  Is the reasult of MD in agreement  with the crystal structure?
How about  using AM1/MM or PM3/MM?  
Thanks a lot!
 
lu