HAR performed at four different levels of theory for each compound. The neutron-diffraction-derived distances are 1.072 (3) and 1.378 (4) Å for 8HQ HMal and 1.1873 (16) and 1.2181 (16) Å for Mg HMal.

8HQ HMal	HF/def2-TZVP	B3LYP/def2-TZVP	B3PW91/def2-TZVP	B3PW91/6-311++G(d,p)
R[F > 4σ(F)]	0.033	0.033	0.033	0.033
wR(F)	0.020	0.019	0.019	0.020
χ2	2.757	2.527	2.500	2.596
δρmax (e Å−3)	0.309	0.331	0.328	0.348
δρmin (e Å−3)	−0.301	−0.289	−0.285	−0.286
δρmean (e Å−3)	0.047	0.047	0.047	0.047
d(O1—H1) (Å)	1.083 (6)	1.042 (8)	1.043 (8)	1.049 (8)
d(O2—H1) (Å)	1.369 (6)	1.403 (8)	1.402 (8)	1.396 (8)