We spent... Waaaay too much time discussing this internally.
As Mrozek notes, compsci is also a common abbreviation for Computer Science. A site for which can be found at http://cs.stackexchange.com/https://cs.stackexchange.com/ - note that we intentionally avoided using the abbreviation for either site, because no matter which one we pick, we'll end up directing some people to the wrong site.
My preference would actually be to call this site "Scientific Computing" and thus keep the abbreviation in line with the name - but Computational Science with "scicomp" as the URL was a compromise solution.
If any change needs to be made though, I would suggest the name of the site rather than the subdomain.