The simplest way to study molecules in quantum chemistry is to study a molecule alone, without any environment effects. The properties of an isolated molecule correspond to its properties in the gas phase, where it does not interact with other molecules. This is why gas-phase calculations are default in all quantum-chemistry programs. Anyway, you may try to calculate the charges both in the gas phase and in water and compare the results.
If you need the charge distribution in the molecule assuming it is dissolved in a certain solvent (not necessarily water), you should set $PCM solvnt=
Sat Feb 8 '14 5:03pm
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Anyway, you may try to calculate the charges both in the gas phase and in water and compare the results.