Prolem about the Charge imbalance in Hydroxyapatite crystals

HP 2015-05-08

First of all ,thank you for your attention to my question.

Recently,I encountered a problem in modeling the Hydroxyapatite with MS.I have got the information about Hydroxyapatite crystals from ICSD as follow:

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I modeled the Hydroxyapatite crystal by using the lattice parameters and the coordinate. Then I do the Geometry Optimization for the model by using Compass Forcefield .And the charges were assigned by the forcefield. However, after the Ceometry Optimization , the charges of the model I got was imbalanced.The details of the charges was shown in the attachment.

In fact, I hope the phosphate radical have the charge of minus three, the hydroxide radicals have the charge of minus 1.

So, how should I do ?