 Hence the CPU requirements of the SHAKE algorithm can become significant, particularly if a molecular model has a high degree of rigidity.A later extension of the method, QSHAKE (Quaternion SHAKE) was developed as a faster alternative for molecules composed of rigid units, but it is not as general purpose.It works satisfactorily for "rigid" loops such as aromatic ring systems but QSHAKE fails for flexible loops, such as when a protein has a disulfide bond.