Quick-start guide

This guide assumes that the PDFgetX3 program has been correctly installed and can be executed by typing pdfgetx3 in a shell window. Please, refer to the installation section if this is not working yet.

The pdfgetx3 program is a command-line application, therefore all the input files and run-parameters are supplied either as command-line arguments or through a configuration file. In general, the pdfgetx3 is executed from a command shell as

pdfgetx3 [options] input1 input2 ... inputN

The inputN stands for an input powder diffraction data. The inputN file is a simple two-column text file, where the first column corresponds to either the 2Θ diffraction angle, or a momentum transfer, Q, in inverse nanometer or inverse ångström units. The second column contains the corresponding X-ray intensities. The input file may start with a header containing comments or metadata related to the measurement. PDFgetX3 will ignore any text leading to a long two-column section. The example input files in this manual were created with the FIT2D program using its “chi” output format, thus we will also refer to them as “chi-files”.

The command-line options are arguments that start with a dash “-” and are used to specify run-parameters or modify the program behavior. The options can be specified in a short form that consists of a dash and a single character, or in a long, more descriptive format starting with a doubled dash --. Options may require values. For short options, the value may be joined to the option string, for example -w0.142774, while for the long options it has to be separated with an equal sign, e.g., --wavelength=0.142774. Although all the PDF calculation parameters can be passed as command line options, it is often more convenient to set them in a configuration file. When run parameter is present both in a configuration file and as command-line option, the command-line value takes precedence. The command-line options are all described in the Options and parameters section of this manual. A brief summary of options can be also displayed by executing

pdfgetx3 --help

The best way of getting familiar with PDFgetX3 is to process the example diffraction data described in the Tutorial. In general, the first step is to create a commented configuration file pdfgetx3.cfg using:

pdfgetx3 --createconfig=pdfgetx3.cfg

The configuration file can have any name, but it is preferable to use either pdfgetx3.cfg or .pdfgetx3.cfg, for these files are automatically loaded by PDFgetX3. All other configuration files must be passed explicitly to the program using the -c, --config option.

Open the pdfgetx3.cfg file in a text editor. The lines that start with a hash mark # are comments and are not used. The lines starting with a right brace [ denote sections in the configuration file. The active lines are all formatted as “NAME=VALUE”. Although PDFgetX3 has many options, in general only a few of them are critical for the PDF calculation:

• dataformat – specifies the input data format
• wavelength – radiation wavelength in Å required for the “twotheta” format.
• composition – chemical composition of the sample
• qmaxinst – upper Q boundary for a meaningful measurement intensities.
• qmax – Q-cutoff for the Fourier transformation that yields the PDF.

Save the updated configuration file and run pdfgetx3 on the input data FILENAME.chi as

pdfgetx3 --verbose=info -t gr FILENAME.chi

Here the --verbose=info option makes pdfgetx3 print more information about its operation. This helps to verify if the configuration file is indeed loaded and if the parameter values are assigned as intended. The PDFgetX3 will not write any output files unless told so. The -t gr option tells the program to save the final G(r) curve as a FILENAME.gr file in the working directory.

The saved .gr file contains a header with all the calculation parameters and the input file name. The .gr file can be therefore also used as a configuration file in order to redo the same calculation

pdfgetx3 -c FILENAME.gr --plot=fq,gr

Note this command does not include any .chi file and this will as a result process the previously used input FILENAME.chi. The --plot=fq,gr option tells PDFgetX3 to display 2 plots for the reduced structure function F(Q) and the final PDF G(r). The --plot option also implies an interactive mode therefore the program does not exit, but starts an interactive IPython session. To exit the interactive mode, type exit() and press Enter.

Please refer to the tutorial section for a step-by step processing of the example files and for demonstration of the PDFgetX3 functions.