Examination of DFT and TDDFT methods I.

We investigated DFT and TDDFT methods in the sense of the molecular orbital (MO) theory and in the framework of the quantum theory of atoms in molecules (QT-AIM). The detailed investigations for the ground state and the pi --> pi* excited state of ethene clarified three aspects about DFT and TDDFT methods: First, the DFT methods included electron correlation effects by directly changing MO energies and MO electron density distributions. Second, MO occupation numbers explained why the delocalization indices (DIs) obtained from DFT wave function files apparently differed from DIs obtained from the conventional correlated wave function files. At last, the significant underestimation of the excitation energy for the pi --> pi* adiabatic excited states of ethene by TDDFT methods can be attributed to the exact degeneracy of HOMO (pi) and LUMO (pi*), a special case of charge transfer (CT) excited states.