Table 4. Quality measures and Fe—O distances (Å) for the four quantum-refined structures with varying protonation states of homocitrate (HCA, cf. Fig. 4 ▸) and different weight factors.

The quality measures are RSZD, RSR and RSCC for homocitrate, the strain energy (ΔE_QM1) and the average difference in the Fe—O bond lengths to the homocitrate O5 (carboxyl­ate) and O7 (alcohol) atoms in the quantum-refined structure (Δd_av), (Fe—O)_QR, and in a structure optimized by QM/MM, i.e. with w_x = 0, (Fe—O)_QM. The best results are marked in bold.

									(Fe—O)QR		(Fe—O)QM
w x	w c	q w	HCA	RSZD	RSR	RSCC	ΔEQM1	Δdav	O5	O7	O5	O7
Auto	1		0H	0.6	0.042	0.969	126	0.13	2.25	2.17	2.18	1.99
∼9			1Hc	0.6	0.043	0.968	168	0.09	2.26	2.18	2.22	2.05
			1Ha	0.6	0.042	0.971	133	0.10	2.23	2.19	2.13	2.29
			2H	0.7	0.043	0.970	120	0.07	2.24	2.19	2.22	2.31
9	1	9	0H	0.8	0.043	0.967	172	0.13	2.26	2.16	2.18	1.99
			1Hc	0.8	0.043	0.969	214	0.07	2.26	2.18	2.23	2.08
			1Ha	0.6	0.042	0.971	159	0.11	2.23	2.18	2.12	2.29
			2H	0.6	0.043	0.970	156	0.07	2.25	2.18	2.22	2.30
1	1	1	0H	2.8	0.055	0.944	38	0.05	2.22	2.06	2.19	1.99
			1Hc	1.7	0.050	0.955	54	0.04	2.23	2.13	2.22	2.06
			1Ha	0.5	0.044	0.967	41	0.03	2.17	2.20	2.13	2.23
			2H	1.1	0.046	0.962	35	0.04	2.22	2.21	2.23	2.28
9	10	0.9	0H	2.6	0.055	0.944	28	0.03	2.21	2.04	2.20	1.99
			1Hc	2.3	0.052	0.951	41	0.04	2.23	2.12	2.23	2.04
			1Ha	0.5	0.044	0.966	33	0.02	2.16	2.20	2.17	2.24
			2H	1.0	0.046	0.964	35	0.05	2.21	2.21	2.22	2.29
1	10	0.1	0H	5.0	0.054	0.945	0	0.00	2.16	1.99	2.16	1.99
			1Hc	4.4	0.063	0.925	7	0.01	2.22	2.07	2.23	2.05
			1Ha	1.5	0.049	0.958	3	0.00	2.13	2.25	2.13	2.25
			2H	2.6	0.051	0.950	1	0.01	2.21	2.28	2.21	2.29