CRILSQ
: Structure factor least-squares refinement 2
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 F
. 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.
Y(o) and Y(c) are 'observed' and 'calculated'
values of F
.
General Structure Parameters
A unit Frel scale factor, set by ADDATM, is applied
and refined.
Overall Thermal Displacement Factor
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.
The Zacchariasen formalism for extinction is
implemented. Use ADDATM for input of the isotropic
extinction parameter (g) and 'ex' as 'apply and refine'
flag.
Absolute-Structure Parameter
The Flack absolute configuration parameter, if
used, must be set by ADDATM. Including 'ab' in the CRILSQ
line flags the 'apply and refine' instruction.
By default all positional parameters are
refined.
Thermal Displacement Parameters
The mode of the thermal displacement parameters is
set by ADDATM. An 'editu' line in that program may
convert the TDP mode.
Occupancy Factor (Population Parameter)
The refinement flag is turned on by 'oc' 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)
Not implemented (yet) are 'Rigid Groups And Identical
Molecules', 'Restrained Parameters' and 'Fixed Atom
Contribution'. The other features are identical to those in
CRYLSQ.
