THE SIR99 PACKAGE: THE VERY LAST PROGRESSES

M.C.Burla3, M.Camalli2, B.Carrozzini1, G.Cascarano1, C.Giacovazzo1, A.G.G.Moliterni1, G.Polidori3, R.Spagna2

1 Istituto di Ricerca per lo Sviluppo di Metodologie Cristallografiche, CNR, Bari, Italy.
2 Istituto di Strutturistica Chimica 'G.Giacomello', CNR, Roma, Italy.
3 Dipartimento di Scienze della Terra, Universita', Perugia, Italy.
E-mail: g.cascarano@area.ba.cnr.it

Keywords: computer program, direct methods

SIR97[1], the heir of Sir92[2], has been recently distributed. It is able to solve up to medium size crystal structures and to completely refine the model structure, providing at the end publication tables and CIF files.

A powerful graphic interface can be used to study the resulting model and is able to guide the user for an efficient and fast structure refinement.

The Sir99 package, not yet distributable, will present new performances:

Restraints
Restraints are incorporated in the least squares refinement as if they were additional experimetal observations. Restraints on distances, angles and on expected planar groups have been introduced. Furthermore, floating origin restraints are generated automatically by the program, when required by the space group symmetry.
Disorder
Sir99 handles structures which contain mixture of different atoms occupying the same site. So far the program uses up to five atomic species and relative occupances for this kind of mixed atom site. A linear restraint of site occupation factors restraints the sum to be constant.
Solution of larger structures
Sir97 was unable to solve medium size structures ( > 200 atoms in asymmetric unit ) owing to inefficiency of the strategy and to memory usage problems. We have improved the procedure. Some examples of large structures ( App, Rubredoxin, …) are solved by the present version of Sir99.
  1. Altomare, A., Burla, M.C., Camalli, M., Cascarano, G., Giacovazzo, C., Guagliardi, A., Moliterni, A.G.G., Polidori, G., Spagna, R. (1998) J. Appl. Cryst., submitted
  2. Altomare, A., Cascarano, G., Giacovazzo, C., Guagliardi, A., Burla, M.C., Polidori, G. & Camalli, M. (1994). J. Appl. Cryst. 27, 435