stead want a more sophisticated 
parametrization, you can write and link your own 
<code><a href="BeamShape.html" target="page">BeamShape</a></code> class.

<p/><code>flag&nbsp; </code><strong> Beams:allowVertexSpread &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Allow the interaction vertex of the two colliding beams to be smeared.
If off, then the vertex is set to be the origin.
  

<p/><code>parm&nbsp; </code><strong> Beams:sigmaVertexX &nbsp;</strong> 
 (<code>default = <strong>0.