Regardless of the coordinate system used in defining the geometry, if XYZ is present, then all atoms will be converted to Cartesian coordinates, and the calculation will be run entirely in Cartesian coordinates.

When a system involving several rings, or big rings, is optimized using internal coordinates, the geometry optimizers often have difficulty.  This is because the effect of a small change in an angle can be a large change in the interatomic distance of two bonded atoms.  This is particularly important in enzymes and other large molecules.  This problem is completely solved if Cartesian coordinates are used.  Therefore, if problems are encountered with geometry optimizations, particularly if internal coordinates are used, keyword XYZ should be used.


See also INT