**RDF**

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 *Q*_{max} 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 3^{rd} 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 4^{th}
value is the number of distance ranges. The 5^{th} 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 (*u*_{0},
*d*_{1}, *
d*_{2}) are then given to
specify *u* for high-*r*. *u*_{0} is the limiting value
of *u*, for very long distances. Note that *
d*_{1} 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/*r*^{2})^{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*, *M*_{1}, *M*_{2},
*q*_{D1}*,
**q*_{D2}*,
k*_{D1},* k*_{D2}) are then given to specify *u*
for high-*r*. *T* is the temperature in Kelvin, *M*_{1}
and *M*_{2} are the atomic masses in amu of the elements involved
in the partial, *q*_{D1}*
*and* q*_{D2}
are the Debye temperatures in Kelvin for the elements involved in the partial,
and *k*_{D1} and *k*_{D2} 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 *M*_{1},=72.59amu,
Debye Temperature *
q*_{D1}=307K,
and Debye wavevector *k*_{D1}=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)*