Author: Roeli Olthof-Hazekamp, Laboratorium voor Algemene Chemie, Padualaan 8, 3508 TB Utrecht, The Netherlands
CRILSQ is a general purpose atomic parameter refinement program. The program can be used for any space group and any setting. It adjusts the scale, extinction, positional, thermal displacement and site occupancy factors by minimizing the difference between observed and calculated structure factors. Refinement is based on F2. Derivatives are only accumulated in full matrix mode. Provision is made for the control of parameters of atoms in special positions and refinement of the Flack absolute parameter XABS. The riding atom model and twin refinement are also supported.
A summary of the method of establishing least-squares equations which are used in CRILSQ can be read in the Program Description of CRYLSQ. A number of the features of CRYLSQ are not implemented. The differences between the two programs are indicated below. New features are the refinement of twin fractions and the calculation of the riding atom model. Most parameters must have been written to the bdf by preceding programs. Their default settings are shown below. 'Apply and refine' flags may be set in the CRILSQ line. Refinement may be shut off by NOREF lines.
The overall isotropic displacement parameter may be set by ADDATM. It may be applied and refined by setting 'ov' in the CRILSQ line. In that case no individual atom displacement parameters are taken into account.
The dispersion parameters are set by STARTX. They are applied, unless a 'nd' flag is present in the CRILSQ line.
Refinement of all parameters in one block (full matrix refinement) is the only option in this program.
The NOREF line may be used for the general parameters SKF, UOV, EXT, ABS and TWI and for the atom specific parameters X, Y, Z, U, U11, U22, U33, U12, U13, U23, UIJ, POP.
The form of the CONSTR lines is the same as in CRYLSQ. The order of the different CONSTR lines is not important.
Hydrogen atom positions should be precalculated in other programs, such as PIG or BONDAT or manually inserted using ADDATM link lines. These programs add H atom link information on to the archive bdf which is read by CRILSQ and used to invoke the riding model. Only H atoms for which linkage has been generated will be treated using the riding model, all other atoms will be refined in the usual manner. The positional parameters of the generated atom is recalculated after each refinement cycle. The isotropic thermal displacement parameter is 1.2 times Uequivalent of the atom to which the new atom is attached.
The twin laws for (pseudo-) merohedral twins will be read from the bdf if placed there by CREDUC, or may be entered by use of 'twin' lines. Up to 8 volume fractions may be refined. Their sum is constrained to 1.0. A 'noref twin' line will apply the volume ratio's, but not refine them. Twin laws should form a sequential, closed group in the sense that: twinop 1 = A1 =I twinop 2 = A2 and A2*A2 = I twinop 3 = A3 and A3*A3 = A2 twinop 4 = A4 and A4*A4 = A2(T)