any CASTEP users or DFT planewave-pseudopotential method users out there? how do I ensure that the basis set is adequate? I'm performing calculations with a rather large unit cell ( orthorhombic, 8 atoms/cell) of a compound AB. Can I justify my choice of energy-cutoff by testing convergence on a smaller cell of AB eg the B2 structure? then using the energy-cutoff that is found adequate for the small cell to do calculations for the orthorhombic cell?