I want to calculate the free energy of reaction at room temperature in DMol3 (as outlined in one of the tutorials). 

Basically, this comes down to calculating the Gibbs free energy of my participating species: a geometry optimization to get the energy at 0K, than a vibrational analysis to get the temperature correction to get the free energy at 298 K.

However, one of my species is a halide-ion, a monoatomic species. Clearly, vibrational analysis on a mono-atomic species is meaningless, yet it should have a entropy contribution to the free energy as well at a finite temperature. I was wondering what the best (consistent) way is to deal with this.