X-PLOR uses a fast Fourier transformation method for the computation of the structure factors and their first derivatives. High efficiency is achieved on supercomputers by the use of special assembly-coded routines.

In the case of CRAY computers, the 3-dimensional FFT calculation is reduced to a series of simultaneous (multiple) 1-dimensional FFT calculations, accomplishing a high degree of vectorization. The complex-to-complex multiple FFT routine CFFTMLT was used, which is available in SCILIB. (SCILIB is a product of CRAY Research Inc.) The routine CFFTMLT requires an initialization, which is provided by the routine CFTFAX. The CFFTMLT routine also requires that the three dimensions of the FFT be multiples of 2, 3, and 5. To avoid memory contingency problems, the physical dimension of the 3-D matrix is increased by one. These requirements are automatically satisified in X-PLOR by the routine FFTPRP, which resides in CRAYTIME.S. However, the user can manually change the parameters by entering the ``XREFin FFT" command level and assigning the appropriate new parameters.

In the case of CONVEX computers, the 3-dimensional FFT calculation is accomplished by using a routine called Z3DFFT that resides in VECLIB. (VECLIB is a product of CONVEX Computer Corp.) VECLIB requires no special provisions during link. The user should check the module CONVEXTIME.S if any problems arise. The Z3DFFT routine requires that the three dimensions of the FFT be multiples of 2, 3, and 5. To avoid memory contingency problems, the physical dimension of the 3-D matrix is increased by one. These requirements are automatically satisified in X-PLOR by the routine FFTPRP, which resides in CONVEXTIME.S. However, the user can manually change the parameters by entering the ``XREFin FFT" command level and assigning the appropriate new parameters.

In the case of SGI computers, special routines are supplied in the directory ``machine/supported/sgi/fft". These routines need to be compiled and linked to X-PLOR. To do so, follow the installation script file.

In the case of all other computers, source code for the Temperton FFT routines is supplied in the corresponding machine-specific module.

Sat Mar 11 09:37:37 PST 1995