The P26 dataset is taken from: "Benchmark database on isolated small peptides containing an aromatic side chain: comparison between wave function and density functional theory methods and empirical force field"
Phys. Chem. Chem. Phys., 2008,10, 2747-2757
DOI: 10.1039/B719294K
The reference structures were taken from:
http://www.begdb.com/index.php?action=oneDataset&id=9&state=show&order=ASC&by=name_m&method=
Optimization level: MP2/cc-pVTZ (RI (Resolution of Identity approximation))