I always have a hard time with the use of on and upon. I have looked at the general comments made in this question, but have the following specific use case in mind that does not seem covered to me: in the sentence we study the influence of anion size on/upon localisation, is one more common than the other?
As pointed out in the answers to the question you posted, the difference lies in the usage, not the meaning. "On" and "Upon" are both correct and would both be understood in the context you present.
The key difference is whether you want your sentence to feel more old-fashioned (and by extension, more formal). In the context of writing a paper about negatively charged particles, one might want to use "on" to prevent said paper from feeling too outdated.
The choice between on and upon in the sentence
We study the influence of anion size on/upon localisation
is not merely a matter of style. Two entirely different meanings are possible.
We study the influence of anion size on localisation
This indicates an investigation into how localisation is affected by anion size.
We study the influence of anion size upon localisation
Although probably not sensible in this chemical context, this sentence could indicate an investigation into the effects or importance of anion size, once localisation has occurred.