I would like to perfrom geometry optimisation on a molecular structure. I thought I should be able to do this in Gaussian or CASTEP, but the calculations cant be ran in materials studio. it is as if the input structure is not compatible (I can run the same calculations no problem when I provide a crystallographic cif structure as my starting structure).

I will be generating a number of potential molecular conformations and I want to run geometry optimisation on them all in order to assess what the most probably conformations are