Expgui reprint
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This document describes EXPGUI, a graphical user interface program for GSAS, a widely used crystallographic software package. EXPGUI was developed to simplify using GSAS for both novice and expert users. It provides an intuitive visual interface as an alternative to GSAS's native text-based interface. EXPGUI displays and allows editing of the GSAS experiment file through a notebook widget with multiple panes corresponding to different parameter sections. It was written using the Tcl/Tk scripting language, making it platform independent and customizable by users.
![using Tcl/Tk.] These Tcl/Tk scripts are not compiled, rather they are
interpreted as the program is run. Since a Tcl/Tk interpreter is
available for all common operating systems, the exact same version of
EXPGUI can be run on all platforms where GSAS is available. The
program has been tested under Windows 95 and Windows NT, as well
as several variants of Unix. Most development is performed under
Linux. Minor adaptations will be needed for the Macintosh; this work
is in progress. AVMS implementation is possible, if interest warrants.
Interpreted languages, such as Tcl/Tk and Perl, run slower than
compiled ones, such as Fortran and C. However, with modern
computers, speed is not usually a critical factor for interactive
programs, since human response times are usually much slower than
programs. Performance of EXPGUI on Unix platforms is excellent.
Windows performance is signi®cantly slower than Unix, but is usually
suf®cient that response is nearly instantaneous.
Interpreted languages offer three advantages. One is that code can
be written and tested in small sections, without compilation. This
speeds program development. Second, since the scripts are distrib-
uted as source code, users can add new features. Indeed, several
sections of EXPGUI were adapted from code submitted to the author
by users. Third, the program can be customized by including code in
user-supplied ®les that are read when the program is started.
2. The EXPGUI interface
The EXPGUI main window is divided into three sections, as seen in
Fig. 1. The menu bar at the top is used to run all of the various GSAS
programs. It is also used to select options within EXPGUI and can be
used to run several utility functions built into EXPGUI.
Below the menu bar is a button bar. The button bar provides a
quick way to invoke frequently used menu bar commands. The
program documentation includes instructions and examples on how
to customize the button bar, so that users can tailor the bar to contain
their own most frequently used options.
The bottom section of the window is used to
display and modify the contents of the experiment
®le. Since the experiment ®le can contain many
different parameters, this space is shared through
use of a notebook widget, and thus there are
`panes' for (i) least-squares options, (ii) phase and
atomic parameters, (iii) diffractogram and back-
ground parameters, (iv) scale factors and phase
fractions, (v) pro®le terms, (vi) constraints on
pro®le terms and atomic parameters, (vii) March±
Dollase model preferential orientation terms, and
(viii) spherical harmonic expansion preferential
orientation terms.
The structure of the experiment ®le to a large
extent determines the layout of each pane. For
example, for each phase in the experiment ®le there
is a set of atomic parameters and (when used) a set
of spherical harmonic terms. Thus the panes
implementing access to atomic parameters and
spherical harmonic terms include a set of buttons
for selecting the phase. Since there is a set of
background terms and diffractometer constants for
each `histogram' (as GSAS names diffractograms),
the panes for those parameters have a list of
histograms to be selected. For some other para-
meters, such as pro®le terms, phase fractions and
March±Dollase preferred orientation terms, a set
exists for every diffractogram, as well as for every phase. For those
parameters, a list of histograms is displayed. When one is selected, the
parameters for all the appropriate phases are displayed.
When GSAS is used with several diffractograms, it is often
convenient to perform a single action that affects parameters for
many histograms, as opposed to repeating the action for each histo-
gram. An example of this is that one may wish to change a set of
diffractometer constants or phase fraction re®nement ¯ags for all
histograms. A special mode in EXPGUI, the multiple-histogram
selection mode, allows the parameters for histograms to be grouped
together for quick access. This can be used by an experienced user
ef®ciently to set re®nement ¯ags or to change initial parameter values
in complex re®nements. EXPGUI has two different types of multiple-
histogram selection modes. One mode allows any combination of
histograms to be selected together. The second set of modes restricts
selection to a particular data type, for example, only CW X-ray data
or only TOF neutron data. When selection of all histograms is
allowed, certain panes are automatically disabled, as it would not
make sense, for example, to group together different types of pro®le
functions or diffractometer constants.
EXPGUI also offers additional modes that affect how data and
results are presented. For example, histograms can be displayed in the
order they are input to GSAS, or can be sorted by angle (wave-
length), bank number or type. Atoms can also be displayed in the
order they are input, or be sorted by atom type, x, y, z, site multi-
plicity, or fractional occupancy. Bond distance and angle output from
the DISAGL program can be displayed in a separate scrollable
window, or can be included in the GSAS listing (Fvƒ„) ®le, as is
normally performed in GSAS.
3. EXPGUI utilities
The program adds additional features to GSAS. Most signi®cant are
two independent Tcl scripts, LIVEPLOT and LSTVIEW. The
J. Appl. Cryst. (2001). 34, 210±213 Brian H. Toby EXPGUI 211
computer programs
Figure 1
A screenshot of the main EXPGUI window.
electronic reprint](https://image.slidesharecdn.com/expguireprint-160715135425/85/Expgui-reprint-3-320.jpg)
Expgui reprint
- 1. electronic reprint Journal of Applied Crystallography ISSN 0021-8898 EXPGUI, a graphical user interface for GSAS Brian H. Toby Copyright © International Union of Crystallography Author(s) of this paper may load this reprint on their own web site provided that this cover page is retained. Republication of this article or its storage in electronic databases or the like is not permitted without prior permission in writing from the IUCr. J. Appl. Cryst. (2001). 34, 210–213 Brian H. Toby ¯ EXPGUI
- 2. computer programs 210 Brian H. Toby EXPGUI J. Appl. Cryst. (2001). 34, 210±213 Journal of Applied Crystallography ISSN 0021-8898 Received 8 December 2000 Accepted 1 February 2001 EXPGUI, a graphical user interface for GSAS Brian H. Toby NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, USA. Correspondence e-mail: brian.toby@nist.gov A description and justi®cation of the EXPGUI program is presented. This program implements a graphical user interface and shell for the GSAS single- crystal and Rietveld package. Use of the Tcl/Tk scripting language allows EXPGUI to be platform independent. Also included is a synopsis of how the program is implemented. 1. Introduction Graphical user interfaces (GUIs) simplify use of computers by presenting information in a manner that allows rapid assimilation and manipulation. The use of visual constructs (widgets) that mimic physical objects such as `switches' and `buttons' can speed learning, by providing an intuitive method to provide input to the computer. A GUI is not always an improvement. As demonstrated by some widely used commercial programs, a poor GUI implementation can obscure functionality. If the GUI is organized in a counterintuitive manner, or if the menu contents are arranged haphazardly, or if commonly performed operations require several unexpected steps to be performed, then a user must typically invest a signi®cant amount of time in learning how to use the program before the program can be used effectively. A good GUI design does not require users to memorize the steps needed to perform an action. This is particularly important for scienti®c applications, where the goal of the user should be to understand the theory behind a program rather than master the arcane steps needed to perform an action. There is one case where memorization may be unavoidable. It is appropriate that a GUI incorporate shortcuts that simplify multi-step tasks; use of these shortcuts may not always be intuitive and expert users may choose to commit them to memory to speed their work. There should also be an obvious way to perform the same tasks, albeit less ef®ciently, without use of the shortcut. This article describes a GUI developed for the Generalized Structure and Analysis Software (GSAS) package (Larson Von Dreele, 2000). The GUI was designed for both novice and expert users, with several goals: to aid scientists who visit the NIST Center for Neutron Research and need to learn Rietveld analysis (Rietveld, 1969), to extend the capabilities of the GSAS software, and to facil- itate research projects, by allowing expert users to work more quickly. 1.1. GSAS The GSAS package has been developed over a period of decades by Allen C. Larson and Robert B. Von Dreele. It is widely used and is arguably one of the most comprehensive packages of crystallographic software ever developed, as it can be used to ®t crystallographic and magnetic structural models to X-ray and neutron single-crystal and powder diffraction data. It can be used with both time-of-¯ight (TOF) neutron and constant-wavelength (CW) neutron powder diffraction data, as well as laboratory and synchrotron CW X-ray powder diffraction data and even energy dispersive X-ray powder diffraction measurements. Up to 99 sets of data may be used in combination to determine a single crystallographic model. In powder diffraction, it is common for a sample to contain more than one crystallographic phase. GSAS allows a crystallographic model to be composed of as many as nine crystallographic phases. In addition to structure determination, GSAS is also used for lattice-constant determination (even for materials with unknown structures), simulation of powder diffraction data, and for texture analysis. Recently, it was enhanced to enable the ®tting of protein structures to powder diffraction data. 1.2. The native GSAS user interface The GSAS package contains approximately ®fty programs. The majority of the programs require minimal or no user input. The only program in the GSAS package that requires extensive user interac- tion is named EXPEDT, an acronym for experiment editor. The EXPEDT program is operated through a series of dialogs, where the user is prompted to answer a question with a series of one-letter options. Typing a question mark (?) causes EXPEDT to print a short explanation for each option. The information provided by the user is saved in a ®le called the experiment (Fiˆ€) ®le. Thus, the experiment ®le contains a wide range of information: for example, atom coor- dinates and diffraction data ®le names, as well as least-squares and Fourier map options. The GSAS package is quite sophisticated, in that a large number of options are available, but this means that the user may have to transcend six or more levels of dialogs in EXPEDT to provide input for a particular option and this can be quite confusing to a novice user. Likewise, it can be very hard to review all the options and values stored in the experiment ®le, as many dialogs must be visited. The EXPGUI program offers an alternate, GUI-based mechanism for reviewing and editing the GSAS experiment ®le. The EXPGUI program implements only a small fraction of the full complement of features available in EXPEDT, but most of the commonly used features for powder diffraction are present in EXPGUI. Since EXPGUI and EXPEDT can be used interchangeably, use of EXPGUI does not prevent access to any capabilities within GSAS. The EXPGUI program can be used in two ways. It can be run from one of the several platform-speci®c `shell programs' that allow access to the various GSAS programs. More signi®cantly, EXPGUI can function both as an experiment ®le editor and as a GSAS shell program, providing GUI access to initiate other GSAS programs. 1.3. Platform independence via Tcl/Tk The EXPGUI program has been written as a series of Tcl/Tk scripts (Ousterhout, 1994). [See Urzhumtseva Urzhumtsev (1997, 1999, 2000) for examples of other crystallographic codes developed electronic reprint
- 3. using Tcl/Tk.] These Tcl/Tk scripts are not compiled, rather they are interpreted as the program is run. Since a Tcl/Tk interpreter is available for all common operating systems, the exact same version of EXPGUI can be run on all platforms where GSAS is available. The program has been tested under Windows 95 and Windows NT, as well as several variants of Unix. Most development is performed under Linux. Minor adaptations will be needed for the Macintosh; this work is in progress. AVMS implementation is possible, if interest warrants. Interpreted languages, such as Tcl/Tk and Perl, run slower than compiled ones, such as Fortran and C. However, with modern computers, speed is not usually a critical factor for interactive programs, since human response times are usually much slower than programs. Performance of EXPGUI on Unix platforms is excellent. Windows performance is signi®cantly slower than Unix, but is usually suf®cient that response is nearly instantaneous. Interpreted languages offer three advantages. One is that code can be written and tested in small sections, without compilation. This speeds program development. Second, since the scripts are distrib- uted as source code, users can add new features. Indeed, several sections of EXPGUI were adapted from code submitted to the author by users. Third, the program can be customized by including code in user-supplied ®les that are read when the program is started. 2. The EXPGUI interface The EXPGUI main window is divided into three sections, as seen in Fig. 1. The menu bar at the top is used to run all of the various GSAS programs. It is also used to select options within EXPGUI and can be used to run several utility functions built into EXPGUI. Below the menu bar is a button bar. The button bar provides a quick way to invoke frequently used menu bar commands. The program documentation includes instructions and examples on how to customize the button bar, so that users can tailor the bar to contain their own most frequently used options. The bottom section of the window is used to display and modify the contents of the experiment ®le. Since the experiment ®le can contain many different parameters, this space is shared through use of a notebook widget, and thus there are `panes' for (i) least-squares options, (ii) phase and atomic parameters, (iii) diffractogram and back- ground parameters, (iv) scale factors and phase fractions, (v) pro®le terms, (vi) constraints on pro®le terms and atomic parameters, (vii) March± Dollase model preferential orientation terms, and (viii) spherical harmonic expansion preferential orientation terms. The structure of the experiment ®le to a large extent determines the layout of each pane. For example, for each phase in the experiment ®le there is a set of atomic parameters and (when used) a set of spherical harmonic terms. Thus the panes implementing access to atomic parameters and spherical harmonic terms include a set of buttons for selecting the phase. Since there is a set of background terms and diffractometer constants for each `histogram' (as GSAS names diffractograms), the panes for those parameters have a list of histograms to be selected. For some other para- meters, such as pro®le terms, phase fractions and March±Dollase preferred orientation terms, a set exists for every diffractogram, as well as for every phase. For those parameters, a list of histograms is displayed. When one is selected, the parameters for all the appropriate phases are displayed. When GSAS is used with several diffractograms, it is often convenient to perform a single action that affects parameters for many histograms, as opposed to repeating the action for each histo- gram. An example of this is that one may wish to change a set of diffractometer constants or phase fraction re®nement ¯ags for all histograms. A special mode in EXPGUI, the multiple-histogram selection mode, allows the parameters for histograms to be grouped together for quick access. This can be used by an experienced user ef®ciently to set re®nement ¯ags or to change initial parameter values in complex re®nements. EXPGUI has two different types of multiple- histogram selection modes. One mode allows any combination of histograms to be selected together. The second set of modes restricts selection to a particular data type, for example, only CW X-ray data or only TOF neutron data. When selection of all histograms is allowed, certain panes are automatically disabled, as it would not make sense, for example, to group together different types of pro®le functions or diffractometer constants. EXPGUI also offers additional modes that affect how data and results are presented. For example, histograms can be displayed in the order they are input to GSAS, or can be sorted by angle (wave- length), bank number or type. Atoms can also be displayed in the order they are input, or be sorted by atom type, x, y, z, site multi- plicity, or fractional occupancy. Bond distance and angle output from the DISAGL program can be displayed in a separate scrollable window, or can be included in the GSAS listing (Fvƒ„) ®le, as is normally performed in GSAS. 3. EXPGUI utilities The program adds additional features to GSAS. Most signi®cant are two independent Tcl scripts, LIVEPLOT and LSTVIEW. The J. Appl. Cryst. (2001). 34, 210±213 Brian H. Toby EXPGUI 211 computer programs Figure 1 A screenshot of the main EXPGUI window. electronic reprint
- 4. computer programs 212 Brian H. Toby EXPGUI J. Appl. Cryst. (2001). 34, 210±213 LIVEPLOT program displays the Rietveld results by plotting an observed and computed diffractogram along with the background curve and the observed-minus-computed differences. As the Rietveld re®nement progresses, the plot is updated. The mouse can be used to magnify (`zoom') a section of the plot by clicking on two corners of the desired region. The LSTVIEW program is used to view the GSAS listing ®le in a scrolled window. Most GSAS programs append output to the end of this ®le. The LSTVIEW program updates the window to include this output as it is written. By default, LSTVIEW scrolls the window so that the most recent re®nement cycle is displayed. Also, agreement factors are highlighted to make it easier to follow the re®nement progress. The original implementation of GSAS in VMS allowed users to keep an archive of previous versions of experiment ®les, so that it was possible to return to a previous stage of a re®nement. This was lost when GSAS was ported to Unix and DOS, but has been reimple- mented in EXPGUI. In Windows, archiving is performed by adding the current experiment ®le to a compressed archive named expnamFs€, where expnam is the root name of the GSAS experi- ment ®le. Compression requires that the shareware PKZIP program be installed in the EXPGUI directory. If this program is not avail- able, the experiment ®le is archived by creating a copy named expnamFiˆ€Fxxx, where xxx is a number between 000 and 999. In Unix, the experiment ®le is copied to a ®le named expnamFiˆ€Fxxx or if the GNU gzip program is available, a compressed ®le named expnamFiˆ€FxxxFgz. Multiple formats are supported for importing coordinates into GSAS, including the Crystallographic Information File (CIF) (Hall et al., 1991). Capitalizing on the interpreted nature of Tcl, as the EXPGUI program is started, it incorporates any ®les named import•XXXXFt™l (where XXXX is an arbitrary name). In this way, a user can implement code to read a new coordinate format by simply creating a new short script. Other utility scripts are incorporated into EXPGUI that (a) convert ®les to or from the appropriate direct-access format needed by GSAS on the particular platform where the program is used, (b) compute the composition of a unit cell, and (c) plot the peak widths as a function of 2. 4. Distribution EXPGUI is available via the Internet from a NIST Web server (ftp:// ftp.ncnr.nist.gov/pub/cryst/gsas/) as well as the Collaborative Computational Project, Number 14 (CCP14) regional mirror sites (Cockcroft, 2000), presently in the UK (ftp://ftp.ccp14.dl.ac.uk/ ccp14/ftp-mirror/briantoby/pub/cryst/gsas/), Canada (ftp:// ccp14.sims.nrc.ca/ccp/ccp14/ftp-mirror/briantoby/pub/cryst/gsas/) and Australia (ftp://ftp.minerals.csiro.au/pub/xtallography/ccp14/ccp/ ccp14/ftp-mirror/briantoby/pub/cryst/gsas/). The code can be down- loaded either as a compressed tar archive (expguiFt—rFgz) or for Windows as a self-expanding zip archive (expguiFexe). The ®les contained in these two distribution archives are identical. GSAS and EXPGUI are also available on CD-ROM via the Crystallographic Nexus project: The International Union of Crystallography, the International Council for Science and the CCP14 collaborate to produce a CD-ROM that is available at no cost to developing world laboratories that lack access to the Internet (Cranswick, 2000). The code is distributed without copyright protection. Thus, it can be modi®ed and redistributed in any fashion. Users are asked to send an e-mail address to the author, so that the program use can be documented and notice of program updates can be distributed. New features are added to the program at irregular but often frequent intervals. As updates are made, revised versions of the program are placed on the Web immediately, as a minimally tested `alpha' version. The updates are available in archive ®les named expgui•ev€reFt—rFgz or expgui•—lph—Fzip. A self-extracting ®le, expgui•—lph—Fexe, is updated less frequently. After the alpha version has been tested for some period of time, it is made the standard version. 5. Documentation Documentation for EXPGUI is provided through more than a dozen Web pages. A Web page is provided to explain the use of each notebook pane. Other Web pages describe menu options, customi- zation options, warning and error messages, and instructions for installation for Unix and for Windows. Context-sensitive help is available in most sections of the program. For example, the F1 key or the `Help on current pane' item in the Help menu, will open a Web browser directed to the appropriate page. Most dialog windows, warning and error messages have a yellow `Help' button. Pressing this button, or the F1 key, opens the documentation providing a more detailed explanation of the error message. Abbreviated help is also available from the `Help summary' item in the Help menu. The Web pages are included in the EXPGUI distribution and are available from a NIST Web server (http://www.ncnr.nist.gov/xtal/ software/expgui), as well as the CCP14 regional mirror sites pres- ently in the UK (http://www.ccp14.ac.uk/ccp/web-mirrors/briantoby/ programs/crystallography/software/expgui), Canada (http://ccp14. sims.nrc.ca/ccp/web-mirrors/briantoby/programs/crystallography/ software/expgui) and Australia (ftp://ftp.minerals.csiro.au/pub/ xtallography/ccp14/ccp/web-mirrors/briantoby/programs/crystal lography/software/expgui). Web pages for the alpha version are posted in a different directory (http://www.ncnr.nist.gov/xtal/ software/expgui/alpha) and are also mirrored by CCP14. 6. Program internals It is convenient to keep the Tcl/Tk scripts in a number of smaller ®les, so that code can be tested more simply and so that some sections of the script are loaded only as they are needed, which speeds initi- alization of EXPGUI. A few of these ®les are worthy of special note, since they are of interest to users wishing to customize or extend the program. The ®le gs—smenuFt™l determines the menu and button bar contents, as well as the functions to be invoked and much of the help summary information. While this ®le can be modi®ed by users, the preferred way to customize this ®le is to include information in a ®le named lo™—l™onfig or ~GFgs—s•™onfig (™X’Fgs—s•™onfig in Windows), as this will override the gs—smenuFt™l contents and will not be overwritten when EXPGUI is upgraded. When the EXPGUI program is running, the contents of the experiment ®le being used is read into a Tcl array named exp—rr—y. The experiment ®le contents are divided into a large number of indexed records, where each record contains a speci®c type of information. Each record is stored in EXPGUI as a separate array element so that it can be accessed via the record's index. A separate array, expm—p, provides lists of which phases, histograms and atoms are in use. After changes have been made to the copy of the experiment ®le in local memory, the contents are written back to the experiment ®le when another program is run or EXPGUI is ended. electronic reprint
- 5. Different routines have been written to extract and change infor- mation in the experiment ®le. As an example, the Tcl command ph—seinfo Q g—mm— is de®ned to return the value for the unit-cell angle and the Tcl command ph—seinfo Q g—mm— set IPH is de®ned to set this angle to 120 . To simplify debugging of EXPGUI, all access to experiment ®le information is made through routines in the ®le re—dexpFt™l. Since these routines simplify reading and changing a GSAS experiment ®le, they may also be of value to anyone wishing to write Tcl programs that interact with GSAS experiment ®les, for example to process a large number of related re®nements on similar data ®les. The EXPGUI program implements a simple mechanism so that changes made on the various input panes are checked for errors, are correctly formatted and are then inserted into the appropriate element in the exp—rr—y array. Each text entry box, checkbutton, etc., is mapped to an element in the Tcl array entryv—r. For example, entryv—r@g—mm—A is used for the unit-cell parameter . For every such array element, a corresponding element in array entry™md provides the Tcl command needed to save this value in exp—rr—y. Thus, every time the variable entryv—r@g—mm—A is changed, the command stored in entryv—r@g—mm—A will be invoked. Values are formatted where needed and this provides a mechanism for error checking. Should an invalid value be speci®ed for a numeric value, for example a string containing letters and numbers, the value is not inserted in array exp—rr—y. Also, the color of the displayed entry is changed to red, to indicate an error is present. Since this array update procedure is performed with every keystroke, it is invoked far more often than is needed, but this operation is fairly rapid compared to human typing speeds, so the impact on perceived program speed is minor. In some program sections, changes are not made on an immediate basis. For parameters such as background terms and atomic and pro®le constraints, editing is performed by creating a new dialog window where values are entered. At the bottom of these windows there are buttons labeled with some variant of `Save' and `Cancel'. Changes are made after `Save' is pressed; no changes are made if `Cancel' is pressed. Error checking of input is made only after the `Save' button is pressed. 7. Conclusions An expandable graphical user interface for the GSAS program suite has been developed. While the interface does not directly support all of the versatility of the entire GSAS suite, it does not preclude use of any GSAS features. The majority of features commonly used with Rietveld re®nements are included; for these features the GUI both aids novice users learning the package and speeds use of GSAS by more expert users. The Tcl/Tk scripting language provides an excellent mechanism for implementing such an interface, since it offers platform indepen- dence, facilitates customization and has a programming environment suitable for rapid program development. Despite the relatively large size of the project (12000 lines of code), the program starts and responds reasonably quickly. I am indebted to Jonathan Wasserman, who coded sections of the initial GUI, and to Dr Pamela Whit®eld, who contributed a draft of the March±Dollase and pro®le constraints sections and reviewed this manuscript. I would also like to thank Lachlan M. D. Cranswick of CCP14, who has made many useful suggestions for improving EXPGUI and has created several Web-based tutorials on the use of EXPGUI, as well as the Engineering and Physical Sciences Research Council of the UK, which funds the CCP14. I am grateful to Dr Diane P. Toby, Dr Whit®eld and Mr Cranswick for reading and commenting on early versions of this manuscript. Many people have contributed indirectly to this effort, through their contributions to Tcl/Tk, but in particular I would like to thank Dr Przemek Klosowski for originally introducing me to Tcl/Tk, Mumit Khan for his work on Windows implementations of gcc, g77 and Tcl/Tk, Michael I. Schwartz for his WINEXEC package, and Dr John K. Ousterhout for creating Tcl/Tk and publicly distributing the source code. Finally, this work could not have been possible without the efforts of Drs Allen C. Larson and Robert B. Von Dreele to create and maintain GSAS. References Cockcroft, J. K. (2000). Chairman, Collaborative Computational Project, Number 14 (CCP14), http://www.ccp14.ac.uk/. Cranswick, L. M. D. (2000). Crystallographic Nexus Software Library on CD- ROM, Daresbury Laboratory, Warrington, WA4 4AD, UK. E-mail: l.cranswick@dl.ac.uk. Hall, S. R., Allen, F. H. Brown, I. D. (1991). Acta Cryst. A47, 655±685. Larson, A. C. Von Dreele, R. B. (2000). General Structure Analysis System (GSAS), Los Alamos National Laboratory, Report LAUR 86±748. Ousterhout, J. K. (1994). Tcl and the Tk Toolkit. London: Addison-Wesley. Rietveld, H. M. (1969). J. Appl. Cryst. 2, 65±71. Urzhumtseva, L. M. Urzhumtsev, A. G. (1997). J. Appl. Cryst. 30, 402±410. Urzhumtseva, L. M. Urzhumtsev, A. G. (1999). J. Appl. Cryst. 32, 376±377. Urzhumtseva, L. M. Urzhumtsev, A. G. (2000). J. Appl. Cryst. 33, 992. J. Appl. Cryst. (2001). 34, 210±213 Brian H. Toby EXPGUI 213 computer programs electronic reprint