2.1.1. Ground-State Benchmarks. Geometries and reference energies for all thermochemical calculations were obtained from the GMTKN55 database, 110 an amalgam of 55 distinct data sets. A description of each data set that is used herein can be found in Table . We group these into three subsets, the first of which consists of reaction barrier heights and includes the data sets BH76, 110 BHPERI, 111 BHDIV10, 110 INV24, 112 BHROT27, 110 PX13, 113 and WCPT18. 114 (See Table for brief descriptions, which are taken from Ref. 110.) The second subset contains basic thermochemical properties for small systems, including reaction energies, and consists of the data sets W4-11, 115 G21EA, 116 G21IP, 116 DIPCS10, 110 PA26, 110,116-118 SIE4x4, 110 YBDE18, 119 AL2x6, 110 NBPRC, 110 ALK8, 110 G2RC, 110 BH76RC, 110 FH51, 120,121 TAUT15, 110 and DC13. The ALKBDE10 123 and HEAVYSB11 110 data sets, which are part of the reaction barrier data in GMTKN55, are excluded here because they contain atoms that are not supported by the def2-TZVPD basis set.