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Natural excitation orbitals as descriptors of single excitations and fractional occupation numbers as descriptors of double excitations in time-dependent response density matrix functional theories

Abstract

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 [3].

References

[1] R. van Meer, O. V. Gritsenko, and E. J. Baerends, Chem. Phys. Lett. 639, 315 (2015).
[2] R. van Meer, O. V. Gritsenko, and E. J. Baerends, J. Chem. Phys. 146, 044119 (2017).
[3] O. V. Gritsenko, submitted.

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