  We have performed fully atomistic classical molecular dynamics (MD)
simulations to calculate the effective interaction between two polyamidoamine
(PAMAM) dendrimers. Using the umbrella sampling (US) technique, we have
obtained the potential of mean force (PMF) between the dendrimers and
investigated the effects of protonation level and dendrimer size on the PMF.
Our results show that the interaction between the dendrimers can be tuned from
purely repulsive to partly attractive by changing the protonation level. The
PMF profiles are well-fitted by the sum of an exponential and a Gaussian
function with the weight of the exponential function dominating over that of
the Gaussian function. This observation is in disagreement with the results
obtained in previous analytic [Macromolecules 34, 2914 (2001)] and
coarse-grained simulation [J. Chem. Phys. 120, 7761 (2004)] studies which
predicted the effective interaction to be Gaussian.

Computational Physics;Soft Condensed Matter;Chemical Physics