Abstract:

We describe a new trail-and-error direct method procedure called STEP. A set of strong reflections, sufficient to solve the structure, is divided into a hierarchy of smaller soluble and connected sub-systems. After having determinable system, we apply three-phase relationship of system to establish the system phase super solution equations. Then, by means of trail and error method, find out phases satisfying the equations.
A new overall figure of merit t XDFOM, has been found to be very effective in picking up a correct solution and the procedure is stopped whenever XDFOM exceeds 4.0 in value.
The program system of STEP, called SYSTEM 90, has been completed, and tested by four difficult known structures and one difficult unknown structure. Satisfying structure models are shown in the E maps. Finally, we mentioned the four SYSTEM90's features which are different from other direct method program. It has been shown that SYSTEM 90 is very powerful to determine difficult structures.

Key words: Crystal structural, Trail and error method, Phase super-solution equation, Refining phase, Figure of merit