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 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.
I think that given the ambiguity between Computational Science (https://scicomp.stackexchange.com/), Computer Science (https://cs.stackexchange.com/) and Theoretical Computer Science (https://cstheory.stackexchange.com/) it is probably a good idea to reserve http://compsci.stackexchange.com/ so that it isn't used by any stack exchange site.
However, if it is going to be reserved, I would like to see that top level page list all three (or more sites in the future) which people might be looking for, with a brief description of the scope and purpose of each site to help them decide.
Incidentally, with regard to the suggestion by Shog9♦ that we change the name of the site from Computational Science to Scientific Computing I would suggest that this is another question entirely.
This alias actually existed when the site first launched. The problem is there's also a Computer Science site, which arguably has a better claim to the name "compsci", so the alias got removed a few days into the public beta. I thought they were going to give it to the Computer Science site when it started, but possibly they're just leaving it undefined to avoid people accidentally going to one site when they meant the other
I requested statistics on how many times compsci.stackexchange.com page has been used. As of June 10, 2019, we are looking approximately at 1000 usages per year.
Now, is 1000 usages per year a lot? I would say no, even if we assume that all of them want to get to Computational Science. Which is certainly not true, since a decent amount of them would want to go to Computer Science or Theoretical Computer Science.
So, the only possible resolution that I can see, is requesting a custom 404 Error page that would list three (?) aforementioned possible websites that the user might have meant.
With 1000 clicks a year, I would have a hard time justifying this development to StackExchange. And if only a third of that (optimistic estimate) were meant to go to our community, that would not likely make any difference for the growth of our community or, even, in the convenience of our existing community.
So, I tend to go for status-declined, unless the Community has a different perspective.