LoRs Chemistry PhD

[jamesjames1888]

I'm applying to Chemistry PhD programs with two strong letters from my previous PIs. However, I'm having trouble choosing a recommender for my third letter.

I'm thinking about asking a professor in a completely unrelated field, as the class was project based and we had a great relationship. The projects were pretty technical and required a lot of coding. Alternatively, I could ask a Chemistry professor whose class I did well in. This professor doesn't know me as well, and there were essentially no larger projects.

The way I see it is that the professor in an unrelated field could comment on my critical thinking, work ethic, and ability to follow though on larger projects. I'm a little worried about asking for a letter from someone in an unrelated field. Please let me know what you think!