'pure' direct method is an ab initio method. Recent advances in ab initio direct methods have enabled the solution of crystal structure of small protein from the type and number of atoms in cell as well as native X-ray date that must be available to atomic resolution, that is, without the use of fragments of known structure or the need to prepare hevy-atom derivatives. An approach that is gaining rapidly in popularity is dual space recycling methods SnB and SHELX-D. They have recently solved the previously known structure of triclinic lysozyme consisting of 1001 non-H protein atoms, none heavier than sulfur. The system optimization method has developed from the integration of systematization, optimization and the probability distribution of structure-invariant. Its program is called SYSTEM-99. On it there is one effective objective function, which is out of ordinary, according to size of the structure, intensity and extensity of relations between reflections, it divides reflections into many subsystems. Then by means of minimization in objective function, it determines all the phases among subsystem in reciprocal space. The SYSTEM-99 program was able to solve the structure of trichosanthes root (TCS) that contains with 1914 non-H protein atoms none heavier than sulfur.

Key words: PROTEIN, CRYSTALLIZE, AB INITIO CALCULATION