I'm not sure if the box is 100 nm or if the particles are 100 nm. If the particles are 100 nm then you have a 300 X 200 nm cluster way above the Rayleigh criterion and, yes, you'll need Mie theory (probably T-Matrix or DDA) to deal with such a cluster.

Thank you sir, 100nm is the Si02 particle radii. I ll see more as your suggest.