Let me be the advocate I never have intended to be; given that I have asked a similar question in the definition phase of this site (How can this proposal be clearly distinguished from Chemistry, Physics, Computational Science, …?).
It is true, that there is, and probably always will be, overlap to other communities. Not only Chemistry, but Physics, Computational Science, Engineering, etc.. I can only speak for Chemistry.se though, because that is where my experience lies.
There we typically seek questions dealing with the concepts of certain aspects; a question ideally is deeply rooted in the chemistry of things. From a computational point of view, we seek questions that will interpret given data, lesser with the focus on how to create such data. We treat computations as a tool to explain the chemistry. Surely, handling of difficult situations during a calculation is equally on topic as how to set up a reaction apparatus there, but when the focus on the chemistry is not obvious, our community tends to view these questions as off topic.
I see some potential that these technical aspects are more on topic here,‡ and that they will find a community happily dealing with them. I believe my question Are support-like queries for software packages on topic? shows this sufficiently.
In any case, this site will certainly create interdisciplinarity amongst the sites that have overlap, but it does that in a focussed way. I especially agree with Geoff, that there really isn't a place for modeling materials, yet.
‡ In what form and to what extend these technical questions will be on topic here is certainly something that still needs to be figured out by the community. There are arguments for leniency, but there are also arguments warning against the abuse of such leniency. (If that is still unclear, I fall into the latter category.) We're still a long way from home, but eventually we will get there.