How to negociate (with a big corp) to be allowed to keep a side-project?

I have a sizeable project I have been working on and off for the last 2 years. It might be commercially viable but it won't make a lot of money.

A few weeks ago, a big corp approached me with an interesting job offer but AFAIK, they are not very tolerant for side-projects. I am wondering how to negociate with them on this.

A few things: - My side project has no connection at all with the corp's market. - the position is in R&D, but the tech has nothing to do with my project. - I have no problem stopping my other consulting activities.