Calculation of EPR g-tensors in the Douglas-Kroll-Kohn-Sham method

An approach to calculate g-tensors of electronic paramagnetic resonance spectroscopy of the sys-tems with doublet spin states based on two compo-nent Douglas-Kroll-Kohn-Sham scheme as imple-mented in the package of quantum-chemical pro-grams ParaGauss is presented. The scheme uses the self consistent two component eigenfunctions of the Hamiltonian with terms directly responsible for the spin-orbit interaction. Therefore g-tensor components are determined as expectation values given by the perturbation theory in the first order with respect to the external magnetic field with the uniform Bko components, and namely g-tensors components are calculated with use of matrix ele-ments of Bko derivatives of theDouglas-Kroll meth-od adopted Zeeman Hamiltonian. In the restricted open-shell Kohn-Sham (ROKS) approximation g-tensor components are given via matrix elements of the of the Kramers doublet relativistic wave func-tions ϕ1 and ϕ2. Results of test calculations for such radicals as CO+, CN, NO2, NF2, HCO, C3H5, TiF3, RhC, PdH are presented. The general fea-tures of measured in EPR spectra g-tensor chemi-cal shifts, directions and relative magnitudes of dif-ferent magnitudes are reproduced. However, for the main groups inorganic molecules g-tensor components are systematically too anisotropic. This is connected with an overestimation of the spin-orbit interaction strengths as a main factor. For im-proved description of the spin-orbit interaction strength the two electron contributions to the matrix transformations of DK scheme connecting real and impulse space constructs should be taken into ac-count. For presently developed calculational scheme theoretical values correlate with measured g-tensor shift with a systematic error corrections.