Using XMD: Entering EAM Potentials Using Tables

Here is a portion of what you are trying to make; an XMD potential file.

#
#  This is an example XMD potential file
#
eunit eV
potential set eam 2
#
potential pair 1 1 10 0.0 5.0
900.0  800.0  700.0           <--- 
600.0  500.0  400.0           <--- -- This is the original table
300.0  200.0  100.0  0.0      <--- 
#
potential dens 1 10 0.0 5.0
900.0  800.0  700.0
600.0  500.0  400.0 
300.0  200.0  100.0  0.0
#
potential embed 1 20 0.0 1000
....

1. Lines beginning with the pound sign (#) are comments, and can be placed anywhere within the file.

2. You should tell XMD what energy units you are using for your potential table. The command is

        EUNIT (unit)
   

   where (unit) can be either eV, ERG, K, JOULE, or

        EUNIT (name) (value)
   

   where (name) is a name that you specify, and (value) is the number of ergs in one of your units.

   You can specify this command more than once, which is useful if your tables do not all use the same energy units.

3. Next, you must tell XMD that you are using an EAM potential and the number of atom types in this potential. The command for this is for this is

        POTENTIAL SET EAM n

   where n is the number of types. Thus if you have an alloy with two component elements, you would use the command

        POTENTIAL SET EAM 2

4. Next, you must specify each EAM function with a different table. If for example you have 2 atoms types, Ni and Al, then there will be 7 EAM functions: two electron density functions (one for each atom type), two embedding functions (again one for each type), and three pair functions (one for Ni-Ni interactions, one for Al-Al, and one cross potential for the Ni-Al interaction).

   In general there are (n² + 5n)/2 EAM functions for n atom types.

   The tables are entered with commands such as

   POTENTIAL PAIR	1 1	2000	1.2 6.3
   POTENTIAL DENS	1	2000	1.2 6.3
   POTENTIAL EMBED	1	1000	0 20.0

   These commands work in the following way.

   • POTENTIAL - Identifies the command as a type of potential command.
   • PAIR, DENS, EMBED - Identifies the table as either a pair potential, electron density of embedding function.
   • (atom type(s)) - Specify to which atom type the function belongs. Note that pair potential functions require two integers, since in the EAM model the pair potentail depends on both the atom types involved. The electron density depends on the type of atom generating the electron density, the embedding function depends on the type of atom receiving the electron density .
   • (table size) - A integer that specifies the number of entries in the table which follow the command.
   • (range) - Two numbers which specify the input range for the table. In the example above, both the pair and electron density functions accept input distances from 1.2 to 6.3 angstroms. For any distance shorter than the allowed range the EAM function will stop the program. For any distance longer than the range (the function cutoff) the function will return zero.
   
   
5. Last comes the table. There number of values in the table must match the number specified above. The first value in the table is the function value at the start of the range - the last value corresponds to the end of the range.

   Thus, for a pair potential or electron density function the last number in the table should be zero, since the function must go to zero at the cutoff. Similarly, for the embedding function the first value in the table must be zero, since it must be zero when the electron density is zero.

6. It is strongly recommended that your table values use the largest number of digits possible. Having too low a precision can cause erratic values for calculated energies and related quantities.

Example