Hi Shaofeng,
Relaxation will give you "a" structure but most likely it is not a representative structure. MD will give you a set of possibly meaningful and representative solvation structures. If you average over these meaningful structures, you can get useful information about the solvation of arsenic ion. Arsenic is anticipated to be ionized in water, so you will need a charged system but Q.E. will automatically apply a neutralizing background. Have a look at some of the early DFT MD work on solvation to get a better idea about what can and cannot be done. For example: http://www.sciencedirect.com/science/article/pii/S0009261497006180 Mostafa Y. MIT
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