RDF

RDF is used to broaden the partial correlation functions of a model for the effects of thermal atomic motion and real-space resolution. It requires a .RDF file as input, and hence it must be run after running the XTAL program.

RDF can only be run from within OpenGenie, and furthermore it will only run successfully if the GEMSQRAW suite of programs has been installed.

To use RDF you must type load "c:\g_f\rdf.gcl" after starting OpenGenie. Then the program will run every time you type rdf.

The algorithm used can in principle only broaden the model correlation functions for the effects of both thermal atomic motion and real-space resolution. If you wish to broaden for real-space resolution only, then set the atomic displacement to zero. It is not possible to broaden for thermal motion only, but if Qmax is set to a large value, say 100Å-1 or greater, then the effect of real-space resolution can be made small.

If more than one different value is required for the atomic displacement, u, then the values must be specified using a .URMS file. Here is an example of a .URMS file for a model with 2 atom types...

5
1 1 1 1 0.16
1 2 1 2 0.05 2.0 0.16
2 1 1 2 0.05 2.0 0.16
2 2 1 1 0.16
0 0 1 1 0.05

The first record gives the number of correlation functions - in this case there are 4 partial functions, and also a total function.

The subsequent records start with origin atom type, and second atom type - note that the partials must follow the order given in the example.

The 3rd value is the broadening mode. Currently three broadening modes are provided, as described below.

In this way, different ranges in r can be broadened with different values of the atomic displacement, u. And different partial functions can be broadened with different values of u too.

The MakeX program can be used to generate a .URMS file for which the RMS bond length variations are calculated from the crystallographic thermal parameters in the .CIF file.

Broadening  mode 1

For mode 1 there are constant values of u, each over a distance range, specified by break-points. The 4th value is the number of distance ranges. The 5th value is the value of u for short distances. This is followed by the value of r for the first break-point. Next comes the value of u for distances longer than the first break-point, and so on.

e.g.
1 2 1 2 0.05 2.0 0.16

For this example, the 1-2 correlation function is broadened with u=0.05Å for distances less than 2.0Å, and then longer distances are broadened with u=0.16Å.

Broadening  mode 2

For mode 2 the low-r region is divided into ranges with constant values of u, the same as for mode 1. Then the final high-r region has an r-dependent value of u, according to the following equation

Three values (u0, d1, d2) are then given to specify u for high-r. u0 is the limiting value of u, for very long distances. Note that d1 is usually set to zero.

If the number of regions is one, then there is no region with a constant value of u.

e.g.
2 2 2 2 0.07 2.5 0.19 0.0 0.1

For this example, the 2-2 correlation function is broadened with u=0.07Å for distances less than 2.5Å, and then longer distances are broadened with u=(0.19–0.1/r2)½.

Broadening  mode 3

For mode 3 the low-r region is divided into ranges with constant values of u, the same as for mode 1. Then the final high-r region has an r-dependent value of u, which is determined according to the correlated Debye model (Jeong et al, Phys Rev B 67(2003)104301)

7 values (T, M1, M2, qD1, qD2, kD1, kD2) are then given to specify u for high-r. T is the temperature in Kelvin, M1 and M2 are the atomic masses in amu of the elements involved in the partial, qD1 and qD2 are the Debye temperatures in Kelvin for the elements involved in the partial, and kD1 and kD2 are the Debye wavevectors in Å-1 for the elements involved in the partial.

If the number of regions is one, then there is no region with a constant value of u.

e.g.
 2 2 3 2 0.05 1.9 293.0 72.59 15.9994 307.0 307.0 1.556 1.556

For this example, the 2-2 correlation function is broadened with u=0.05Å for distances less than 1.9Å, and then longer distances are broadened with a thermal width , where the mean square displacement of element 1 is calculated according to the correlated Debye model, with temperature T=293K, mass M1,=72.59amu, Debye Temperature qD1=307K, and Debye wavevector kD1=1.556 Å-1.

A version list for RDF is here.

Last Updated 02 Sep 2010 by Alex Hannon (a.c.hannon@rl.ac.uk)