There are a lot of different elements implemented in CalculiX, therefore it is not always easy to select the right one. In general, one can say that the quadratic elements are the most stable and robust elements in CalculiX. If you are not a finite element specialist the use of quadratic elements is strongly suggested. This includes:
Other elements frequently exhibit unsatisfactory behavior in certain instances, e.g. the C3D8 element in bending states. Unless you are a specialist, do not use such elements. Detailed information is given underneath.