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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><HTML><HEAD> <META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9"> <TITLE>XMD - Molecular Dynamics Program: Interatomic Potentials</TITLE> <LINK HREF="xmd-12.html" REL=next> <LINK HREF="xmd-10.html" REL=previous> <LINK HREF="xmd.html#toc11" REL=contents></HEAD><BODY><A HREF="xmd-12.html">Next</A><A HREF="xmd-10.html">Previous</A><A HREF="xmd.html#toc11">Contents</A><HR><H2><A NAME="s11">11. Interatomic Potentials</A></H2><P><P>At this point in time, the POTENTIAL command is incompletely documented.The recommended way to implement a potential is to obtain an existingpotential file that will make the necessary calls to the POTENTIALcommand. Contact the maintainer of XMD for more information (JonRifkin, jon.rifkin@uconn.edu).<P><H2><A NAME="ss11.1">11.1 Using Provided Potentials</A></H2><P><P><H2><A NAME="ss11.2">11.2 Using Tersoff's Carbon-Silicon Potential</A></H2><P><BLOCKQUOTE><CODE><PRE># Set constants for Tersoff's C-Si potential# (11 Nov 1996)#potential set tersoffdtime 0.8e-15calc DTIME=0.8e-15calc C=1calc Si=2calc MassC=12.01calc MassSi=28.086calc A0=4.32eunit ergecho off## Data about this potential## Si A0 at 0K = 5.432 ang# Si B at 0K = 0.979e12 erg/cm^3## Experimental values# Si A0 at 0K = 5.451# Si B at 0C = 0.979e12 erg/cm^3#echo on</PRE></CODE></BLOCKQUOTE><P><H2><A NAME="ss11.3">11.3 Using Stillinger-Webers Silicon Potential</A></H2><P><BLOCKQUOTE><CODE><PRE># Set constants for Stillinger-Weber Si potential# (12 Feb 1998)#potential set stillcalc DTIME=0.5e-15dtime DTIMEcalc Si=1calc MassSi=28.086calc A0=5.428eunit erg</PRE></CODE></BLOCKQUOTE><P><BLOCKQUOTE><CODE><PRE></PRE></CODE></BLOCKQUOTE><P><BLOCKQUOTE><CODE><PRE></PRE></CODE></BLOCKQUOTE><P><H2><A NAME="ss11.4">11.4 Creating EAM Potentials</A></H2><P>In practice most EAM potentials, from whatever source, are stored astext tables. This document describes how to adapt a text table for usewith XMD. The only program this requires is a text editor. Once you haveconverted the text table to an XMD readable format, you should store thepotential in its own file. This way it can be easily accessed from anyXMD input (using the READ command). <P><P>Here is a portion of what you are trying to make; an XMD potential file. <P><BLOCKQUOTE><CODE><PRE>## This is an example XMD potential file#eunit eVpotential set eam 2#potential pair 1 1 10 0.0 5.0900.0 800.0 700.0 <--- 600.0 500.0 400.0 <--- -- This is the original table300.0 200.0 100.0 0.0 <--- #potential dens 1 10 0.0 5.0900.0 800.0 700.0600.0 500.0 400.0 300.0 200.0 100.0 0.0#potential embed 1 20 0.0 1000....</PRE></CODE></BLOCKQUOTE><P><OL><LI>Lines beginning with the pound sign (#) are comments, and can be placedanywhere within the file. <P></LI><LI>You should tell XMD what energy units you are using for your potentialtable. The command is <BLOCKQUOTE><CODE><PRE>EUNIT (unit)</PRE></CODE></BLOCKQUOTE>where (unit) can be either eV, ERG, K, JOULE, or <BLOCKQUOTE><CODE><PRE>EUNIT (name) (value)</PRE></CODE></BLOCKQUOTE>where (name) is a name that you specify, and (value) is the numberof ergs in one of your units. <P>You can specify this command more than once, which is useful if yourtables do not all use the same energy units. <P><P></LI><LI>Next, you must tell XMD that you are using an EAM potential and thenumber of atom types in this potential. The command for this is for thisis <BLOCKQUOTE><CODE><PRE>POTENTIAL SET EAM n</PRE></CODE></BLOCKQUOTE>where n is the number of types. Thus if you have an alloy with twocomponent elements, you would use the command <BLOCKQUOTE><CODE><PRE>POTENTIAL SET EAM 2</PRE></CODE></BLOCKQUOTE><P></LI><LI>Next, you must specify each EAM function with a different table. If forexample you have 2 atoms types, Ni and Al, then there will be 7 EAMfunctions: two electron density functions (one for each atom type), twoembedding functions (again one for each type), and three pair functions(one for Ni-Ni interactions, one for Al-Al, and one cross potential forthe Ni-Al interaction). <P>In general there are (5n + n^2)/2 EAM functions for n atom types. <P>The tables are entered with commands such as <BLOCKQUOTE><CODE><PRE> atom types table size range ---------- ---------- ---------POTENTIAL PAIR 1 1 2000 1.2 6.3 POTENTIAL DENS 1 2000 1.2 6.3 POTENTIAL EMBED 1 1000 0 20.0 </PRE></CODE></BLOCKQUOTE><P><P>These commands work in the following way. <UL><LI>POTENTIAL - Identifies the command as a type of potential command. </LI><LI>PAIR, DENS, EMBED - Identifies the table as either a pair potential,electron density of embedding function. </LI><LI>(atom type(s)) - Specify to which atom type the function belongs. Notethat <B>pair potential functions require two integers</B>, since in the EAM model the pair potentail depends on both the atom typesinvolved. The electron density and embeding function only depend on theatom type generating the electron density (the DENS function) or theatom type receiving the electron density (the EMBED function). </LI><LI>(table size) - A integer that specifies the number of entries in thetable which follow the command. </LI><LI>(range) - Two numbers which specify the input range for the table. Inthe example above, both the pair and electron density functions acceptinput distances from 1 to 6.3 angstroms. For any distance shorter thanthe allowed range the EAM function will stop the program. For anydistance longer than the range (the function cutoff) the function willreturn zero. </LI></UL><P><P></LI><LI>Last comes the table. There number of values in the table must match thenumber specified above. The first value in the table is the functionvalue at the start of the range - the last value corresponds to the endof the range. </LI></OL><P><P>Thus, <B>for a pair potential or electron density function the last numberin the table should be zero, </B>since the function must go to zero at the cutoff. Similarly, <B>for the embedding function the first value in the table must be zero,</B>since it must be zero when the electron density is zero. <P><P><P><P><HR><A HREF="xmd-12.html">Next</A><A HREF="xmd-10.html">Previous</A><A HREF="xmd.html#toc11">Contents</A></BODY></HTML>⌨️ 快捷键说明
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