  The dispersion component of the van der Waals (vdW) interaction in
low-dimensional metals is known to exhibit anomalous "Type-C non-additivity"
[Int. J. Quantum Chem. 114, 1157 (2014)]. This causes dispersion energy
behavior, at asymptotically large separations, that is missed by popular
atom-based schemes for dispersion energy calculations. For example, the
dispersion interaction energy between parallel metallic nanotubes at separation
$D$ falls off aymptotically as approximately $D^{-2}$, whereas current
atom-based schemes predict $D^{-5}$ asymptotically. To date it has not been
clear whether current atom-based theories also give the dispersion interaction
inaccurately at smaller separations for low-dimensional metals.
  Here we introduce a new theory that we term "MBD+C" . It permits inclusion of
Type C effects efficiently within atom-based dispersion energy schemes such as
Many Body Dispersion (MBD) and Universal MBD (uMBD). This allows us to
investigate asymptotic, intermediate and near-contact regimes with equal
accuracy. (The large contact energy of intimate metallic bonding is not
primarily governed by dispersion energy and is described well by semi-local
density functional theory.) Here we apply a simplified version,"nn-MBD+C", of
our new theory to calculate the dispersion interaction for three
low-dimensional metallic systems: parallel metallic chains of gold atoms,
parallel Li-doped graphene sheets; and parallel (4,4) armchair carbon
nanotubes. In addition to giving the correct asymptotic behavior, the new
theory seamlessly gives the dispersion energy down to near-contact geometry,
where it is similar to MBD but can give up to 15% more dispersion energy than
current MBD schemes, in the systems studied so far. This percentage increases
with separation until nn-MBD+C dominates MBD at asymptotic separations.
