Département de chimie moléculaire

While most applications of the linear response formulation of time-dependent d.-functional theory (TDDFT) have been to the calcn. of the excited states of mols. with closed-shell ground states, Casida's formulation of TDDFT opened the way to TDDFT calcns. on mols. with open-shell ground states by allowing for different-orbitals-for-different-spin, Although a no. of publications have now appeared applying TDDFT to mols. with open-shell ground states and give surprisingly good results for simple excitations, it is relatively easy to show that some excited states of open-shell mols. will have unphys. large amts. of spin contamination. There is thus a clear need for computational tools which can sep. phys. from unphys. excited spin states in TDDFT. We address this need by using analytic deriv. techniques to develop formulas for the 1- and 2-electron reduced d. difference matrixes, in essential agreement with those obtained by Rowe in the equation-of-motion superoperator approach to Green's functions in nuclear physics. The corresponding formula for excited-state spin contamination appears to be generally good enough for assigning excited-state spin symmetries, but does lead to a small overestimation of S {%} 2 in the cases considered here. This (apparently small) problem is eliminated when the Tamm-Dancoff approxn. is used in TDDFT. [on SciFinder(R)]