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

M. G. DEL PÓPOLO, C. PINILLA, AND P. BALLONE

JOURNAL OF CHEMICAL PHYSICS

AMER INST PHYSICS

Año: 2007 vol. 126 p. 1447051 - 14470511

0021-9606

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 = 0K 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 (~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.