2D and 3D lattices, crystals, and materials in general are completely on topic in my opinion!
As for your concern about DFT being on-topic at ChemSE, here I will cite Tyberius who knows the ChemSE very well and has about 10,000 reputation there:
From my own experience with Chem, computational questions are still
fairly niche there, often having difficulty attracting the
specialized, expert attention they would need.
So far we have about 20 questions/day here:
And we are just getting started (it's only been 1 day and only 40% of the followers from Area51 have had a chance to join Beta so far; plus we will not be advertising everywhere until it goes to Public Beta, since a lot of us are academics that don't want to gain a negative reputation for spamming people with sites that turn out not to be the "real thing").
We are planning to ramp up to 100 questions/day in the coming weeks.
The DFT tag on Chem SE has only 6 questions/month, and 32 questions this year:
Here at this site, we already have 10 DFT questions in the first day and a half, even though most people do not even know about our site yet and we do not show up on the main SE page or in Google Searches while in "Private" Beta:
So expert-level DFT questions will probably get more attention from high-level experts here.
Finally, I will repeat what I wrote at the end of this answer on the topic of overlapping SE sites:
If I may cite what Rory Alsop recently said on the Drones SE chat, the idea is that if a question overlaps with multiple SE sites, then it is the asker's choice where to put the question if they think the question would be well received on a particular site, and the community/mods can help suggest a migration to a different site if it turns out that the question might get a more valuable response somewhere else:
Finally I would like to cite users Mark and Catija from the DronesSE Meta site, when asked whether or not a certain type of question was on topic: