Molecular replacement: the revival of the Molecular Fourier transform method

The molecular Fourier transform method, perhaps the first application of th e molecular-replacement approach, used in the 1950s for the two-dimensional structure determination of small molecules, has been modernised for the ef ficient solution of complex structures. In the modern application of the mo lecular Fourier transform (MFT), the three-dimensional transform of the mol ecular model is calculated and fitting is achieved by rotating the weighted reciprocal lattice with respect to the calculated transform. The fit betwe en the transform and the weighted reciprocal lattice is gauged by three dif ferent criteria corresponding to R factor, correlation coefficient and prod uct function. Since the procedure involves the rotation of indices and is, therefore, independent of the number of atoms, it is much faster than other methods which employ the rotation of the molecular model. This feature ena bled the renovation of the rotation-translation search method ULTIMA, which utilizes low-order data and packing considerations for the efficient solut ion of large structures.