First-principles calculations of long-range intermolecular dispersion forces

Sammanfattning: This work presents first-principles calculations of long-range intermolecular dispersion energies between two atoms or molecules as expressed in terms of the C6 dipole-dipole dispersion coefficients. In a series of publications, it has been shown by us that the complex linear polarization propagator method provides accurate ab initio and first-principles density functional theory values of the C6 dispersion coefficients in comparison with those reported in the literature. The selected samples for the investigation of dispersion interactions in the electronic ground state are the noble gases, n-alkanes, polyacenes, azabenzenes, and C60. It has been shown that the proposed method can also be used to determine dispersion energies for species in their respective excited electronic states. The C6 dispersion coefficients for the first π → π' excited state of the azabenzene molecules have been obtained with the adopted method in the multiconfiguration self-consistent field approximation. The dispersion energy of the π → π' excited state is smaller r than that of the ground state. It is found that the characteristic frequencies ω1 defined in the London approximation of n-alkanes vary in a narrow range and that makes it possible to construct a simple structure-to-property relation based on the number of -bonds for the dispersion interaction in these saturated compounds. However, this simple approach is not applicable for the interactions of the π-conjugated systems since their characteristic frequencies ω1 vary strongly depending on the systems.