For what kind of molecules and crystals can CASTEP provide reliable NMR predictions? Linear response properties (magnetic shielding and J-coupling) can be calculated for diamagnetic molecules and solids. Electric field gradients can be computed for a wider range of systems (metals and non-metals, diamagnetic and paramagnetic materials)
I completed a NMR ab initio simulation, now how can I compare it with my experimental spectra?
A completed NMR simulation with CASTEP will produce in output a .magres file. You can download and use MagresView (http://ccpforge.cse.rl.ac.uk/gf/project/magresview/) to visualize its contents on a 3D model of the molecule (for example, labelling the various atoms with their respective chemical shifts). For simple systems and experiments this could be enough to compare with your spectra. For the more complex cases, you might want to use a simulation program like Simpson (available for free at http://nmr.au.dk/software/simpson/) to reproduce the spin dynamics in your experiment. In this case, MagresView and its utilities can still help you by producing input files ready to be submitted to Simpson for a quick spectrum simulation.