Improvements in DFT Calculations of Spin–Spin Coupling Constants

Abstract

Different types of spin-spin coupling constants (SSCCs) for several representative small molecules are evaluated and analyzed using a combination of 10 exchange functionals with 12 correlation functionals. For comparison, calculations performed using MCSCF, SOPPA, other common DFT methods, and also experimental data are considered. A detailed study of the percentage of Hartree-Fock exchange energy in SSCCs and in its four contributions is carried out. From the above analysis, a combined functional formed with local Slater (34%), Hartree-Fock exchange (66%), and P86 correlation functional (S66P86) is proposed in this paper. The accuracy of the values obtained with this hybrid functional (mean absolute deviation of 4.5 Hz) is similar to that of the SOPPA method (mean absolute deviation of 4.6 Hz).

Assessment of DFT functionals with fluorine–fluorine coupling constants

“…This basis set ensures the correct cusp behaviour of the wave function and consequently a very good description of the FC term [20]. The aug-cc-pVTZ-J basis set yields SSCCs close to those obtained with larger basis sets [20,[27][28][29]. This work is based on the indicated B3LYP/aug-cc-pVTZ geometries.…”

“…However, an additional set of optimised MP2/6-31+G(p.d) geometries has been used to analyse the geometry effect on these SSCCs and on these molecules. Functionals used in this work can be sorted in three groups: (1) the first group is a selection formed by B3LYP [22,23], PBE1PBE [30,31], TPSSTPSS [32], B972 [33], B97D [34] (equivalent to B97 [35,36]); functionals from Head-Gordon et al [37,38] wB97, wB97X and wB97XD; a set of functionals developed by Handy et al [39][40][41][42] HCTH (HCTH407), HCTH147, HCTH93, tHCTH and tHCTHhyb including a functional HCTH147 with 20% of HF exchange, named in this work as HCTH147-20; three Minnesota functionals [43,44], M06, M06L, M062X; and four functionals (HFS41, S47LYP, S55VWN5 and S66P86) recently optimised to calculate SSCCs [29]. (2) The second group was proposed to analyse the exchange and the correlation functional effects.…”

“…It should be noted that due to the small number of SSCCs for each type of couplings, the statistic parameters presented as well as the functional order based on these parameters cannot be considered in a strict way. Furthermore, mare statistic, that could provide interesting information when the samples present different magnitudes, may be distorted if some of the magnitudes are close zero [29]. Note also that vibrational corrections and solvent effects on these fluorinefluorine SSCCs have not been considered in this initial study.…”

“…Geminal and vicinal fluorine-fluorine SSCCs present the largest rmse and mae values and also the largest mare with the exception of 5 J FF SSCCs which yield the largest mare values. Functionals recently tested for SSCCs [29] are among the best for geminal and to a lesser extent for vicinal couplings. These functionals (S55VWN5, S47LYP, S41 and S66P86) have high HF exchange contribution.…”

“…[29]. The SSCCs for the data set studied in this work were calculated with 11 different exchange functionals using in all of them the LYP correlation functional.…”