Local and semilocal density functional computations for crystals of 1-alkyl-3-methyl-imidazolium salts

• Published in: The Journal of chemical physics Vol. 126; no. 14; p. 144705
• Main Authors: Del Pópolo, M G, Pinilla, C, Ballone, P
• Format: Journal Article
• Language: English
• Published: United States 14.04.2007
• ISSN: 0021-9606
• DOI: 10.1063/1.2715571

The accuracy and reliability of popular density functional approximations for the compounds giving origin to room temperature ionic liquids have been assessed by computing the T=0 K crystal structure of several 1-alkyl-3-methyl-imidazolium salts. Two prototypical exchange-correlation approximations have been considered, i.e., the local density approximation (LDA) and one gradient corrected scheme [PBE-GGA, Phys. Rev. Lett. 77, 3865 (1996)]. Comparison with low-temperature x-ray diffraction data shows that the equilibrium volume predicted by either approximations is affected by large errors, nearly equal in magnitude (approximately 10%), and of opposite sign. In both cases the error can be traced to a poor description of the intermolecular interactions, while the intramolecular structure is fairly well reproduced by LDA and PBE-GGA. The PBE-GGA optimization of atomic positions within the experimental unit cell provides results in good agreement with the x-ray structure. The correct system volume can also be restored by supplementing PBE-GGA with empirical dispersion terms reproducing the r-6 attractive tail of the van der Waals interactions.