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<H3>surf_modify command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>surf_modify group-ID keyword args ... 
</PRE>
<LI>group-ID = ID of the surface group to operate on 

<UL><LI>one or more keyword/arg pairs may be listed 

<LI>keyword = <I>collide</I> or (react) 

<PRE>  <I>collide</I> arg = sc-ID
    sc-ID = ID of a surface collision model
  <I>react</I> arg = sr-ID
    sr-ID = ID of a surface reaction model or <I>none</I> 
</PRE>

</UL>
<P><B>Examples:</B>
</P>
<PRE>surf_modify sphere collide 1
surf_modify all collide sphere react sphere 
</PRE>
<P><B>Description:</B>
</P>
<P>Set parameters for a group of surface elements in the specified
group-ID.  Surface elements are read in by the
<A HREF = "read_surf.html">read_surf</A> command.  They can be assigned to groups by
that command or via the <A HREF = "group.html">group</A> command.
</P>
<P>The <I>collide</I> keyword is used to assign a surface collision model.
Surface collision models are defined by the
<A HREF = "surf_collide.html">surf_collide</A> command, which assigns each a surface
collision ID, specified here as <I>sc-ID</I>.
</P>
<P>The effect of this keyword is that particle collisions with surface
elements in group-ID will be computed by the surface collision model
with <I>sc-ID</I>.
</P>
<P>The <I>react</I> keyword is used to assign a surface reaction model.
Surface reaction models are defined by the
<A HREF = "surf_react.html">surf_react</A> command, which assigns each a surface
reaction ID, specified here as <I>sr-ID</I> or the word "none".  The latter
means no reaction model.
</P>
<P>The effect of this keyword is that particle collisions with surface
elements in group-ID will induce reactions which are computed by the
surface reaction model with <I>sr-ID</I>.  If "none" is used, no surface
reactions occur.
</P>
<P>Note that if the same surface element is assigned to multiple groups,
using this command multiple times may override the effect of a
previous command that assigned a different collision or reaction model
to a particular surface element.
</P>
<P><B>Restrictions:</B>
</P>
<P>All surface elements must be assigned to a surface collision model via
the <I>collide</I> keyword before a simlulation can be performed.  Using a
surface reaction model is optional.
</P>
<P>This command cannot be used before surfaces exist.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "read_surf.html">read_surf</A>, <A HREF = "bound_modify.html">bound_modify</A>
</P>
<P><B>Default:</B>
</P>
<P>The default for surface reactions is none.
</P>
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