Noble gas endohedral fullerenes, Ng@C60 (Ng=Ar, Kr): a particular benchmark for assessing the account of non-covalent interactions by density functional theory calculations
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This work is dedicated to a special test, checking the capability of density functional theory computations in the account of long-range effects. The particular case of endohedral noble gas atoms in C60 fullerene puts several methodological challenges, such as the numeric problem in the balance of covalent versus non-covalent interactions. We designed a procedure based on the shifts of vibrational frequencies in C60 versus Ng@C60 couples. The energy scale of the investigated vibrations is comparable to that of the extracted van der Waals interaction parameters, achieving then a well-tempered description. A phenomenological model, based on transparent analytical formulas of the totally symmetric modes, is outlined and used to assess the computational results on series of functionals belonging to different classes (genuine forms, long-range corrected and with empiric dispersion ingredients). While the vast majority of the tested functionals undergo failures, good results are obtained for some long-range-corrected functionals (LC-BLYP and LC-wPBE), which follow the Tsuneda and Hirao’s scheme, provided that richer basis sets (with diffuse components) are used. Successes are obtained also for the Grimme B97D functional, when coupled with the D2 and D3 dispersion scheme, the results being rather independent from the basis set, as expected from the empirical nature of this type of amendment.
KeywordsDensity functional theory Long-range effects Non-covalent interactions Vibration Hamiltonian Endohedral fullerenes
The authors acknowledge financial support from the Romanian Ministry of Education and Research through the UEFISCDI/CNCS research grant PN2-ID-PCE-14/2013. FC and AS acknowledge travel grant support from Roumanian Academy and CNR Italy.
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