Table 3. Main-chain average bond lengths in the structure 1lri after QM and conventional refinements.

Bond	Non-regional QM	Regional QM	Conventional	Engh & Huber†	Ultrahigh resolution‡
N—Cα	1.44	1.45	1.45	1.46 (2)	1.45 (1)
C—O	1.25	1.24	1.23	1.23 (2)	1.23 (1)
Cα—C	1.54	1.53	1.52	1.53 (1)	1.53 (2)
C—N	1.36	1.34	1.33	1.34 (2)	1.33 (2)
Cα—Cβ §	1.54/1.53	1.53/1.53	1.52/1.53	1.54 (3)/1.53 (2)	—

^†The ideal Engh and Huber parameters (Engh & Huber, 1991 ▶).

^‡Statistics derived from ultrahigh-resolution protein structures (Jaskolski et al., 2007 ▶).

^§The first value corresponds to Ile, Thr and Val residues; the second value corresponds to the remaining amino acids excluding Ala.