Van der Waals versus optical interaction between metal nanoparticles.

We derive closed expressions for the Casimir-Polder potential between metal nanoparticles as well as for the light-induced interaction owing to the gradient force. Within the validity of the dipole approximation, the maximum interaction energy turns out to be proportional to the plasma frequency, and it is comparable to the thermal energy at T=300 K. On the other hand, the light-induced interaction depends linearly on the light intensity, and only for strongly focused laser beams (10-100 mW/microm(2)) does it become comparable in strength to the Casimir-Polder interaction.