On Short-Range Enhancement of Van-der-Waals Forces
  We show that for nanoparticles with giant polarizability $\alpha$ (a prominent example being the recently studied Na$_{14}$F$_{13}$ molecular clusters) van der Waals forces are significantly enhanced at interparticle distances shorter than $R_0=(\alpha/2\pi\epsilon_0)^{1/3}$. For an adequate description of this phenomenon, nonlinear effects must be taken into account. We show that, contrary to some theoretical claims, an accurate treatment of nonlinearity does not lead to any repulsive forces. 
K. Makhnovets and A. Kolezhuk
cond-mat.mes-hall cond-mat.other