ding in their CM frame, 
and therefore only the CM energy needs to be provided, see 
<code>Beams:eCM</code> below.
  
<br/><code>option </code><strong> 2</strong> : the beams are back-to-back, but with different energies,
see <code>Beams:eA</code> and <code>Beams:eB</code> below.
This option could also be used for fixed-target configurations.
  
<br/><code>option </code><strong> 3</strong> : the beams are not back-to-back, and therefore the
three-momentum of each incoming particle needs to be specified, see