  We assess a variant of linear-response range-separated time-dependent
density-functional theory (TDDFT), combining a long-range Hartree-Fock (HF)
exchange kernel with a short-range adiabatic exchange-correlation kernel in the
local-density approximation (LDA) for calculating isotropic C6 dispersion
coefficients of homodimers of a number of closed-shell atoms and small
molecules. This range-separated TDDFT tends to give underestimated C6
coefficients of small molecules with a mean absolute percentage error of about
5%, a slight improvement over standard TDDFT in the adiabatic LDA which tends
to overestimate them with a mean absolute percentage error of 8%, but close to
time-dependent Hartree-Fock which has a mean absolute percentage error of about
6%. These results thus show that introduction of long-range HF exchange in
TDDFT has a small but beneficial impact on the values of C6 coefficients. It
also confirms that the present variant of range-separated TDDFT is a reasonably
accurate method even using only a LDA-type density functional and without
adding an explicit treatment of long-range correlation.
