Hi @fabian358,

You need to use the _S_MaterialModel property of the SolveData object:

materialModel = surface.MaterialCell.CreateSolveType(ZOSAPI.Editors.SolveType.MaterialModel)._S_MaterialModel
materialModel.IndexNd = 1.33
materialModel.AbbeVd = 5
surface.MaterialCell.SetSolveData(materialModel)

If you want an easier way to set the material model (and other solve types), you can check out ZOSPy, which reduces this to a single function call:

import zospy as zp

...

zp.solvers.material_model(
    surface.MaterialCell, 
    refractive_index=1.33, 
    abbe_number=5
)