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
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
Here is a portion of what you are trying to make; an XMD potential file.
# This is an example XMD potential file
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.
Lines beginning with the pound sign (#) are
and can be placed anywhere within the file.
You should tell XMD what energy units you are using for
your potential table. The command is
where (unit) can be either
EUNIT (name) (value)
where (name) is a name that you specify,
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.
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
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
These commands work in the following way.
Identifies the command as a type of potential command.
Identifies the table as either a pair potential,
electron density of embedding function.
- (atom type(s)) -
Specify to which atom type the function belongs.
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.
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.
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.
Last updated March 8, 2000.