NWChem/QA/tests/dft_bsse

========= Dummy centers vs Ghost centers ==========
==== The Characters
Originally dummy centers (named "x") were a mathematical trick to specify 
zmatrices for tricky molecules.  E.g. benzene, or linear molecules.  
Because of their role it is alright to delete dummy centers. This was 
introduced at some point to clean up zmatrices. After deleting the dummies 
the autoz function would be used to regenerate a zmatrix. 

Ghost centers (named "bq" after the ghost Banquo from William 
Shakespeare's Macbeth) were introduced to calculate BSSE 
free energies. These centers have basis sets, but no charge and 
no electrons.

==== The conflict
The way the deletion is implemented means that different things happen 
depending on how dummy centers are named.  Due to a different choice 
of internal coordinates this could lead to a different optimization path.  

==== The resolution
To fix this all centers starting with "x" but not "xe" are deleted and a new 
zmatrix generated. Bq centers by their nature should not be deleted, and they 
now won't be.

==== The dénouement
Not clear to me at present is what happens if the original input 
zmatrix contains constraints. I don't see how these can be handled so that they 
would always survive the elimination of the dummy centers. Of course one could 
replace the dummy centers by ghost centers without a basis set to deal with  
such a case. The dft_bsse test case has been updated to match the new approach.