Fixing coordinates

 Dear colleagues,
 does anyone know how to fix two atoms on an axis (e.g. x-axis)
 cartesians coordinates of the whole molecule (containing more than 3
 I am using babel to convert from one input format to another and I have
 the above problem on translating the coordinates.
 Thank you in advance,
 Nick Kourkoumelis
 University of Ioannina - Greece
 e-mail: me00007 ( ( at ) )