Table 4 Comparison of errors between experimental and OQMD-predicted formation energies for a variety of material classes, under different sets of elemental chemical potentials

Category	$〈 Δ H_{f}^{EXP} 〉$	Number of comps.	Fit-none		Fit-partial		Fit-all
			Err	MAE	Err	MAE	Err	MAE
Magnetism
Magnetic	−1.231	327	0.135	0.151	0.052	0.102	0.023	0.097
Non-magnetic	−1.234	1343	0.098	0.132	0.011	0.093	−0.005	0.076
Bandgap
Metallic (E_g=0)	−0.706	921	0.056	0.098	0.031	0.086	0.005	0.077
Semi-conductor (0<E_g<2)	−1.104	249	0.137	0.155	0.017	0.115	0.007	0.093
Wide bandgap (E_g>2)	−2.175	491	0.180	0.195	0.003	0.101	−0.006	0.083
Number of components
Binary	−0.935	1376	0.089	0.124	0.025	0.097	0.002	0.082
Ternary	−2.187	259	0.179	0.191	0.002	0.094	0.002	0.082
Quaternary	−2.439	33	0.207	0.214	0.019	0.085	0.020	0.071
Bonding type
I–VII (ionic)	−2.225	18	0.224	0.224	0.090	0.090	0.052	0.057
III–V (covalent)	−0.577	15	0.144	0.146	0.095	0.123	0.056	0.090
Intermetallic binary (metallic)	−0.434	278	0.006	0.071	−0.002	0.069	−0.013	0.069
Binary compounds that contain a/an: ^a
Alkali metal	−1.047	87	0.120	0.126	0.052	0.081	0.017	0.056
Alkali earth metal	−1.256	92	0.161	0.191	0.098	0.128	0.009	0.092
Transition metal	−0.727	855	0.051	0.094	0.002	0.079	−0.006	0.074
f-block metal	−1.106	414	0.098	0.128	0.060	0.098	0.010	0.074
Semi-metal ^b	−0.573	317	0.046	0.093	0.027	0.093	−0.009	0.082
Post-transition metal ^c	−0.651	233	0.078	0.101	0.028	0.093	−0.005	0.075
Halide	−1.717	258	0.199	0.214	0.015	0.135	0.014	0.122
Compounds w/o corrections ^d	−0.514	780	0.031	0.081	0.031	0.081	−0.009	0.066
Overall	−1.232	1670	0.105	0.136	0.020	0.096	0.002	0.081

Abbreviations: comps, compounds; Err, average error; MAE, mean absolute error; OQMD, Open Quantum Materials Database; w/o, with out.
The definitions of the different fits are given in the section ‘Formation Energy Calculation’. Average experimental formation energies ( $〈 Δ H_{f}^{EXP} 〉$ ), Err and MAE are given in eV/atom.
^aAt least one of the components in the binary compound belongs to a given subcategory of elements, and the second component can be any element.
^bWe take semimetal elements to be B, Si, Ge, As, Sb and Te.
^cWe take post-TM elements to be Al, Ga, In, Sn, Tl, Pb and Bi.
^dIncludes all compounds that do not contain any elements that require GGA+U or chemical-potential corrections.

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