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Re: Partial charges calculations (RHF/6-31G(d)) - in gas or water?


Dear Anna,

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.
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= $END. Although water is the default solvent name when PCM is switched on (by the presence of $PCM group), it is not the only solvent available in FireFly. Chemists frequently study molecules in nonaqueous solvents, and many nonpolar solvents (such as hydrocarbons) only slightly affect molecular properties, so gas-phase calculation can be a good approximation in this case. This is the other reason why $PCM with solvnt=water is not switched on by default.

Anyway, you may try to calculate the charges both in the gas phase and in water and compare the results.

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