DIFFICULTIES IN THE CALCULATION OF ELECTRON-SPIN-RESONANCE PARAMETERSUSING DENSITY-FUNCTIONAL METHODS

Density functional theory is applied to the calculation of the isotrop ic hyperfine coupling constants in some small molecules. Various funct ionals are tested. The agreement of the calculated values to experimen tal data and values obtained from sophisticated ah initio methods depe nds on the functionals used and the system under consideration. With r espect to spin density calculations the functional of Lee, Yang and Pa rr with Becke's exchange functional (BLYP) is found to give good resul ts for the heavier center of the CH and the NH molecule, while the spi n densities of other molecules such as OH, H2CN, H2CO+, NO and O-2 dev iate considerably from experimental and/or other theoretical results ( 30%-60%). In cases where the singly occupied orbital can contribute to the isotropic hyperfine coupling constants, accurate results are obta ined. The reason for this is analyzed.