Overlap between computer science and chemistry? This isn't particularly a question about chemistry itself, but more-so the prospect of chemistry. 

What kind of opportunities arise when you overlap chemistry and computer science? I'm a computer science major, but prior to going to university I had (and still have) a deep passion for chemistry. My highest marks were always in chemistry, but I decided to go down the compsci route because likewise it is a passion of mine. An opportunity has presented itself to double major in compsci and chemistry and I'm curious to know whether it's worth it.

My knowledge of chemistry is fairly barebones (a limitation of highschool education), but as I said, it was my strongest subject and for some reason it just clicks with me. I'd often spent sleepless nights perusing chemistry textbooks and online periodicals because it fascinates me to no end.

I'm not looking for greater job prospects (I already work in both compsci as a programmer and chemistry in an electronic cigarette company), I'm more-so looking for a more fulfilling pursuit of knowledge. While irrelevant, my university recently opened a new building dedicated primarily to chemistry, so facilities are recent and faculty is adept and education is modern. I'd very much like to take advantage of the opportunity, but I'm just unsure if a university-level chemistry program will scratch that itch for me.
Have a look at list of computational chemistry software. It gives you overview, how the computers are used in chemistry. Many of them fall more into the chemical physics field, some into the chemoinformatics, to name just few. So definitely there are lot of places to contribute to use of computers in chemistry. If you are able to mix the software engineering and the science itself in a good way, possibilities are immense.

As for the last question - almost whatever field of chemistry you choose, you can contribute to the programs used within it. There is no such thing like completed scientific code. And university level is a necessary prerequisite to do reasonable science. Just concentrate on the models and mathematics behind the scenes and everything will follow.