I am not sure if I need Lumerical FDTD or Lumerical MODE. We know Lumerical MODE has a Var-FDTD solver. What are the cases when Var-FDTD cannot be used?

In the following case you can not use Var-FDTD: 

1- Fiber optics cannot be simulated since the structure is not planar.

2- Structures with graded refractive index in the x or y directions cannot be simulated since smoothly varying refractive index within the propagation plane, which is the xy plane,

is not supported, although if the refractive index is graded along the z-dimension, this

can be simulated.

3- Waveguides with sloped sidewalls, also cannot be simulated as the structure cross section

is smoothly-varying in the propagation plane.

4- Grating couplers which couple light into or out of the xy plane in the vertical direction

cannot be simulated since the light should be travelling in-plane.

5- Nanoparticles which scatter light in 3 dimensions cannot be simulated for the same reason that the method does not support light propagating out of the xy plane.

6- Grid attributes, such as permittivity rotation, np density or temperature grid attributes

imported from DEVICE simulations are also not supported.