International Workshop on 'New challenges in Reduced Density Matrix Functional Theory: Symmetries, time-evolution and entanglement'
Natural excitation orbitals as descriptors of single excitations and fractional occupation numbers as descriptors of double excitations in time-dependent response density matrix functional theoriesOleg Gritsenko
Physical interpretation is given to the elements of the matrix diagonalization problem of time-dependent response Hartree-Fock (TDHF) and density matrix functional (TDDMFT) theories. A simple description of single excitations calculated in TDHF and TDDMFT is given in terms of natural excitation orbitals (NEOs) [1,2]. The NEOs are obtained by diagonalization of a submatrix in the diagonalization problem and they closely resemble virtual Kohn-Sham orbitals of density functional theory (DFT). A crucial role of fractional occupation numbers (FONs) of the natural orbitals (NOs) in description of double excitations obtained in TDDMFT is demonstrated. An analytical expression for the energy of a double excitation as a function of the FON ratio is derived in a simple model .
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 O. V. Gritsenko, submitted.