Many have commented here on the computational challenges of enumerating, storing, and searching very large virtual libraries. While molecules can be represented and stored as strings, that's an oversimplification of the problem (from a CS perspective). Scientists often want to search these large libraries in 2D and 3D, which requires computing & storing those coordinates. It can be cost prohibitive just to store the 3D coordinates for massive virtual libraries, even for large pharmaceutical companies. We've done this for 10^10 molecules [0]. If you are reading this and find this type of problem interesting, checkout www.eyesopen.com/careers
[0] https://pubs.acs.org/doi/10.1021/acs.jcim.9b00779 "Virtual Screening in the Cloud: How Big Is Big Enough?"