Nevermind, I’m completely oblivious. It’s clearly described in the Methods. I’m glad I caught this before I sent out the email.
Local sequence–structure compatibility
To evaluate the compatibility between the local sequence and the local structure, we collected 200 fragments for each nine-residue frame in the designed sequence from a non-redundant set of X-ray structures based on the sequence similarity and secondary structure prediction22 (the standard Rosetta fragment generation protocol for ab initio structure prediction). Then, for each frame, we calculated the root mean squared deviation between the designed local structure and each of the 200 fragments. Designs were ranked on the basis of the total number of fragments for which the root mean squared deviation was less than 1.0 Å, and those with high values were selected.