Dispersion energy in density-functional theory
Date
2019
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Density Functional Theory (DFT), in various local and semilocal approximations,
cannot completely describe long-range correlations between the electrons responsible
for dispersion interactions. A large number of methods have been designed
to correct DFT for the missing dispersion effects (DFT+D methods). These methods
add a fraction of true dispersion energy to DFT methods assuming that a part of it has
already been recovered by DFT. We estimate the amount of dispersion recovered by
different popular DFT methods and show that what appears to be recovered dispersion
energy does not possess the physical character expected of dispersion interactions.
Moreover, a large part of it originates from those terms of the DFT interaction energy
that do not have any physical mechanism to capture such effects. The technique used to estimate the recovered dispersion will help for future developments of DFT methods as it points out the shortcomings of the dispersionless parts of the DFT interaction energy as well. A new method for calculating dispersion interactions is also developed using a modified polarizability density from nonlocal correlation methods. The performance of the new method is tested on a set of dimers at various intermonomer separations. The
new method outperforms all nonlocal correlation functionals and reduces the average
error on the test set by at least a factor of 2. Finally, a path for the future development
of nonlocal correlation methods is provided by comparing polarizability densities from
nonlocal correlation functionals to the accurate one provided by time-dependent DFT.