Center for Materials Simulation

University of Connecticut
Institute of Materials Science

Using XMD: Entering EAM Potentials Using Tables

In practice most EAM potentials, from whatever source, are stored as text tables. This document describes how to adapt a text table for use with XMD. The only program this requires is a text editor. Once you have converted the text table to an XMD readable format, you should store the potential in its own file. This way it can be easily accessed from any XMD input (using the READ command).

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
The beginning of an example XMD potential file.

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

  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.

  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.


You can view an example EAM potential file. This is a file for a cobalt potential developed by S. Chen at the Center for Materials Simulation.

Maintained by
Last updated March 8, 2000.