I am trying to model a system that has both anions and cations interacting with each other. (In this case it is calcium ion interacting with two small carboxylic acid molecules). Now, looking through literature, I have found many people dealing with such systems using augmented basis sets on all atoms.
However, most introductory molecular modelling books say that augmented basis functions should only be used with anions, and atoms that are participating in long-range interactions. It is also frequently mentioned that using diffuse functions on cations can cause basis set overcompleteness problem.
So what is the best choice and why? All augmented, or mixed, or no diffuse functions?