DIRDIF -TRACOR

TRACOR

TRACOR is a program to find the position of a molecular fragment (with correct orientation) by means of reciprocal space correlation functions. The input fragment is used for the calculation of partial structure factors for all reflections (for the entire expanded data set). The partial structure factor (sum of partial structure factors of symmetry related fragments) depends on the shift vector t. The correlation between calculated and observed structure factors (intensities) determines the best value for the shift vector t. The actual calculations are done by the Fast-Fourier-Transform method, and employs all symmetry elements simultaneously.

The program is used for expanding structural fragments with correct orientation but unknown position. The program is automatically called after the execution of ORIENT. If the 'best' solution from ORIENT fails to solve the structure, however, the user may call for TRACOR and supply the fragment rotated by the angular settings of the second ORIENT solution.

Note that a correctly oriented fragment sometimes is available as the result of a failure of ab-initio direct methods; when a recognizable fragment does not allow expansion or refinement, then the fragment may be misplaced, however the orientation is certainly correct, and the user may call for TRACOR. The program is also a powerful tool for the elucidation of heavy atom structures. For instance, the origin and the next largest non-HARKER Patterson peak define a pair of heavy atoms which can be used as a well oriented model to be positioned by the program TRACOR.

When the user calls for TRACOR, the system will automatically arrange for the following calculations: first expand the reflection data and calculate structure factors (program DDMAIN sub FCALC), then find the position of the fragment (program TRACOR), and find the remainder of the structure (programs PHASEX and FOUR, recycling).