Hi,

Before running the actual single-point or geometry optimization calculations on the configurations with RUNCONFS, a list of configurations needs to be generated by use of the CONFRAND option. The list of generated configurations is saved into a file CONFIGURATIONS.DAT that is then read by the next RUNCONFS calculation.

Let us go through this step-by-step. I take your system as an example.

First, you would setup an input for the CONFRAND calculation. For instance:

Title
CRYSTAL
0 0 0
194
3.065 17.656
4
22 0 0 0.5
14 0 0 0.75
22 0.666666 0.333333 0.364919
6 0.333333 0.666666 0.427507
SCELCONF
1 0 0
0 1 0
0 0 1
CONFRAND
1
5
2
END

Here I am selecting just one crystallographic site for substitution, specified by atom number 5, which is a Ti atom. Given the symmetry of this system, that atom has a multiplicity of 4 (i.e. there are other 3 Ti atoms symmetry-related to it). This can be inspected from here (in bold the selected atoms, in italic its symmetry-equivalents):

 N. ATOM EQUIV AT. N.          X                  Y                  Z

   1   1   1   22 TI    0.00000000000E+00  0.00000000000E+00 -5.00000000000E-01
   2   1   2   22 TI    0.00000000000E+00  0.00000000000E+00  0.00000000000E+00

   3   2   1   14 SI    0.00000000000E+00  0.00000000000E+00 -2.50000000000E-01
   4   2   2   14 SI    0.00000000000E+00  0.00000000000E+00  2.50000000000E-01

 **5   3   1   22 TI   -3.33334000000E-01  3.33333000000E-01  3.64919000000E-01**
  *6   3   2   22 TI    3.33334000000E-01 -3.33333000000E-01 -1.35081000000E-01
   7   3   3   22 TI    3.33333000000E-01 -3.33333000000E-01 -3.64919000000E-01
   8   3   4   22 TI   -3.33333000000E-01  3.33334000000E-01  1.35081000000E-01*

   9   4   1    6 C     3.33333000000E-01 -3.33334000000E-01  4.27507000000E-01
  10   4   2    6 C    -3.33333000000E-01  3.33334000000E-01 -7.24930000000E-02
  11   4   3    6 C    -3.33333000000E-01  3.33334000000E-01 -4.27507000000E-01
  12   4   4    6 C     3.33334000000E-01 -3.33333000000E-01  7.24930000000E-02

The last input parameter of CONFRAND, which I set to 2, determines how many of these 4 Ti atoms will be substituted.

By running it you get the following output:

*******************************************************************************
  SUBSTITUTIONS AT SITES (LABELS) : 
    5    7    6    8
  **********************************            COMPOSITION :          2 /   4
  **********************************          NUMBER OF SIC :                3
 *******************************************************************************
 --->              1      SIC  FOUND AT TRY              1   -   CONFIGURATION 
   0   0   1   1
   MULTIPLICITY      2   -   RANK           1   -   CANONICAL RANK           1
 --->              2      SIC  FOUND AT TRY              2   -   CONFIGURATION 
   1   0   1   0
   MULTIPLICITY      2   -   RANK           5   -   CANONICAL RANK           2
 --->              3      SIC  FOUND AT TRY              4   -   CONFIGURATION 
   1   0   0   1
   MULTIPLICITY      2   -   RANK           3   -   CANONICAL RANK           4
 *******************************************************************************
                   3      SIC  FOUND  AFTER              4       TRIES
 *******************************************************************************

that is 3 symmetry-independent configurations (SICs) are found and stored in the external file CONFIGURATIONS.DAT.

At this point you are ready to run a RUNCONFS calculation (note that the CONFIGURATIONS.DAT file generated at the previous step needs to be placed inside the scratch folder of the new job). For instance with:

Title
CRYSTAL
0 0 0
194
3.065 17.656
4
22 0 0 0.5
14 0 0 0.75
22 0.666666 0.333333 0.364919
6 0.333333 0.666666 0.427507
SCELCONF
1 0 0
0 1 0
0 0 1
RUNCONFS
ATOMSUBS
22 273
END

In this case I ask to substitute Ti with Ta.

Hope this helps,

Alessandro Erba