Intermolecular dispersion energies from coupled exact-exchange Kohn-Sham excitation energies and vectors

Abstract

The calculation of intermolecular dispersion energies within a time-dependent density-functional theory framework (TDDFT) is reviewed. While the commonly used route to compute dispersion energies is to employ the Casimir-Polder integral transform and thereby describing the dispersion energy as a functional of the response functions of the monomers, this alternative approach leads to a particularly simple form for the long-range interaction between two subsystems in terms of the TDDFT eigenvectors and excitation energies. We present two different schemes to reduce the high computational cost for calculating the excitation energies and vectors of the monomers to be used to compute the dispersion energies via the TDDFT method. This is achieved by a decoupling of the occupied-virtual orbital product basis functions which are associated with single-particle excitations belonging to large and small oscillator strengths. It will be shown that this approach can lead to large speedups of 90% and more compared to a full diagonalisation of the hessian, while the fully coupled dispersion energies can be reproduced with a reasonable accuracy. We also investigate the role of the dispersion energy for the description of σ- and π-stacking interactions and discuss various dispersion energy approximations, including the D3 model by Grimme.