4. UFO library-structured files and (x,y) data tables

Many of the files in the RTAB database utilize a simple library-structured file format. With this format, multiple logical files (or blocks of information) can be tightly associated in a single physical file. The start of a logical file within a UFO-structured library file (Kissel et al., 1991a,b) is designated by a record of the form

      *BLOCK:identifier

where the asterisk ('*') starts in the first column and 'identifier' is a string of characters used as a unique identifier for the block of information that follows.

For example, the specification 'data_SCF/047_scf0sl' indicates a file named '047_scf0sl' within the folder (directory) 'data_SCF' in the RTAB database. This file contains the Dirac-Slater self-consistent potential for neutral Silver (Ag, Z=47). Within this physical file is contained the following logical files

      *BLOCK:PARAMETERS
      *BLOCK:SUMMARY
      *BLOCK:CONFIG
      *BLOCK:V

This file structure makes it relatively simple for a program (or a human) to locate specific information within the file, and it is used extensively by the elastic scattering codes.

The UFO-specification also defines a mechanism for explicitly referring to these blocks of information by means of extensions to the normal filename, and utility subprograms within the RAYLIB support library implement this capability. For example, a user might specify

      047_scf0sl|module=V

as the filename for an elastic-scattering program requiring access to the self-consistent potential for Ag. The vertical bar ("|") is used to separate UFO extensions from the normal filename. In some situations, it may be necessary for the filename to be enclosed in quotes, or for the vertical bar to be "escaped" (e.g., "\|" on Unix systems), to avoid misinterpretation by the computer.

Keywords in UFO extensions can be abbreviated, and multiple non-conflicting extensions can be specified with a single filename. The reference to the potential for Ag above could be abbreviated to

      047_scf0sl|m=V

Utility subprograms within the RAYLIB support library allow for the flexible reading of (x,y) data tables prepared in the UFO data format. In addition to simple tables of (x,y) data pairs, more complicated data tables are supported where, for example, the specific columns of a larger table can be selected as the x- and y-data (the independent and dependent variables, respectively), or the data can be transformed after being read from the file.

Specific examples of UFO extensions that will be encountered by a user of the FFTAB code (discussed subsequently) include:

      013_mf0sl|m=TOTAL ,

specifies the total-atom modified relativistic form factor for Aluminum (Al, Z=13), selected from a library-structured file that contains separate modules for the total atom ("|m=TOTAL"), individual shell ("|m=K", "|m=L", etc.) and individual subshell ("|m=L1", "|m=L2", etc.) form factors;

      006_asf0sl|m=RE|y=3 ,

specifies the real anomalous scattering factor f' for Carbon (C, Z=6) for use with the relativistic form factor (as opposed to g' for use with the modified relativistic form factor) from a library file that separately tabulates the real ("|m=RE") and imaginary ("|m=IM") parts, selecting column 3 ("|y=3") for the y values (by default, "|x=1" and "|y=2" - in this example, "|y=2" would select g');

      082_nf0h75|t=0.01220*y ,

computes the non-relativistic form factor per electron for Lead (Pb, Z=82) by scaling the existing values in the file by 1/Z ("|t=0.01220*y" specifies a transform whereby the y values read from the file are multiplied by the factor 0.01220 before the values are returned to the calling program). [footnote - The nonrelativistic form factors of Hubbell et al. (1975) are stored in their original form wherein f(0)=N, where N is the number of electrons in the atom. The FFTAB code needs the form factor per electron (the default form of the modified relativistic form factor, g, and relativistic form factor, f, computed by Kissel).]

Many more transformations and extensions are implemented, and the interested reader should consult the UFO documentation for details (Kissel et al., 1991a,b).

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Information date: Sep. 2, 2000 lk