Typically, SE sites decide their topicality independently from each other. But knowing Chemistry.SE a little bit, I am thinking about, what is the topic of the site, what would not be on topic on there?
I believe, simply finding a single, characteristic topic part, which is obviously off-topic on Chemistry.SE, but is on topic here, would already create the identity of the site.
Sorry it's taken me a long time to write an answer.
The question of how we differ from ChemSE came up in August 2019 in the first week of our Area51 journey, and I provided this answer.
Adding to that I will mention one other way in which we differ:
This question about Frank Jensen's basis sets was posted 1 month ago on ChemSE and got 0 answers, 0 votes, and has 65 views (many of them coming recently because the question was re-asked on this SE, with a link to the original question).
The user re-asked here:
And within 1 day got an answer from Frank Jensen himself!
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.
I think the creation of a Materials Modeling SE apart from the Chemistry.SE is working well, at least for me. Besides the question mentioned by @Nike Dattani, I had other two questions brought here that got proper answers or helpful comments afterwards. For around four other modelling questions I got answers in Chemistry.SE and didn't need to migrate them here, so I owe a big thanks to people there as well.
I haven't had much time engaging as an active member of StackExchange. I started fewer than two months ago, in Chemistry.SE, so my opinion is based on limited experience, but my impression is that modelling/software questions have mixed reception there. Maybe not everyone sees modelling as proper chemistry, due to the classical popular depiction of a chemist as a whitecoat-clad person, working in a wet lab, instead of a silicon lab, which is unfortunate, because both are valid ways of doing chemistry.
I think eventually this separation in people's minds will eventually dissolve, perhaps when most schools discover computers are good pedagogical tools in chemistry and apply them. For example, I personally think it's easier to explain stereoisomerism using a software like Avogadro as a visualization aid, instead of just drawing skeletal formulas in the blackboard and using your hands for an analogy. Until that day arrives, I think it's a good idea to have a dedicated Stack Exchange, so the questions on materials modeling receive focused attention, instead of getting diluted and some ending forgotten.
I feel lucky to have created my account just around the time materials modelling launched into beta.
I would see the site dropping in nicely alongside physics.se and chemistry.se. Computational materials science, including both solid state physics and quantum chemistry, is the third cornerstone of modern science. Experiments and theory have been complemented by simulations. The name of the site could maybe be adjusted to clarify this: I would prefer having the word "computational" somewhere in there.
The real nice thing about this is that the site already seems to have attracted people from both chemistry and physics, and both the molecular and crystalline side of things. This is a very good sign for the future.
