Why does {p-But-calix[4]-(OMe)2(O)2ZrCl2} distort away from C2v symmetry?

The distortion away from the C2v symmetry observed in the X-ray structure of the {p-But-calix[4]-(OMe)2(O)2ZrCl2} complex is explained with the help of density functional calculations on the Zr(OH)2(H2O)2Cl2 model system. The distortion is not caused by intermolecular packing effects but rather by the steric constraints imposed upon the O-Zr-O bond angles by the attachment of the oxygen atoms to the calix[4]arene ligand.