Table 1 Comparison of the lattice parameters and unit cell volume for the P2/₁a structure of melamine as obtained from experimental data and semi-local plane-wave DFT predictions utilising different DFT-D formulations.

Method	a (Å)	b (Å)	c (Å)	\(\alpha \) (°)	\(\beta \) (°)	\(\gamma \) (°)	Volume (Å\(^3\))
SCND⁴⁰ (14 K)	10.433	7.458	7.238	90.0	113.30	90.0	517.25
HRPND (–d₆) ³³ (15 K)	10.448	7.456	7.243	90.0	113.37	90.0	517.97
PBE-TS [0.07 1/Å]	10.592	7.409	7.152	90.0	113.28	90.0	515.54
PBE-TS [0.05 1/Å]	10.566	7.411	7.166	90.0	113.21	90.0	515.70
PBE-TS [0.02 1/Å]	10.566	7.411	7.166	90.0	113.21	90.0	515.70
PBE-TS-SCS [0.05 1/Å]	10.832	7.352	7.282	90.0	109.89	90.0	545.30
PBE-MBD@rsSCS [0.05 1/Å]	10.645	7.321	7.170	90.0	111.64	90.0	519.33
PBE-D4 [0.05 1/Å]	10.533	7.380	7.231	90.0	112.24	90.0	520.17
PBE-D3(BJ)-ATM [0.05 1/Å]	10.573	7.382	7.253	90.0	111.86	90.0	525.37
PBE-D3(BJ) [0.05 1/Å]	10.573	7.382	7.253	90.0	111.87	90.0	525.37
PBE-D3(0) [0.05 1/Å]	10.625	7.398	7.253	90.0	111.82	90.0	529.28
PBE-D2 [0.05 1/Å]	10.329	7.368	7.147	90.0	113.74	90.0	497.86
PBE [0.05 1/Å]	11.459	7.370	7.626	90.0	104.05	90.0	624.76
HRPND (–d₆) ³³ (297 K)	10.583	7.479	7.283	90.0	112.27	90.0	533.38
SCXRD³⁹ (300 K)	10.606	7.495	7.295	90.0	112.26	90.0	536.68
SCXRD³⁶ (300 K)	10.537	7.477	7.275	90.0	112.90	90.0	527.99
PBE-TS AIMD 300 K [0.05 1/Å]	10.686	7.496	7.247	90.0	113.21	90.0	533.47

The experimental data were obtained from single-crystal neutron (SCND)⁴⁰ and x-ray (SCXRD)^36,39 diffraction along with high-resolution time-of-flight neutron powder diffraction (HRPND) experiments³³. The finite-temperature structural parameters obtained from AIMD simulations are compared with the room-temperature experimental data^33,36,39. The k–point spacing given in square brackets [in 1/Å] defines the accuracy of the Brillouin zone sampling, which converges with the density mesh of 0.5 1/Å. The unit cell dimensions are in the units of Å. The cell volume is tabulated in Å\(^{3}\).

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