CCL: Banquo atoms Vs. Dummy atoms

 Sent to CCL by: "Johannes Hachmann" [jh388(_)cornell.edu]
 Hi Anthony,
 the terms 'ghost atoms' and 'dummy atoms' are sometimes used
 interchangeably, but there are two distincted meanings:
 'Dummy atoms' are often used as auxillary coordinate points for the
 specification of molecular geometries, constrained geometry optimizations,
 or use of molecular symmetry (think e.g. of a benzene z-matrix where there
 is no real atom at the inversion point), when this is otherwise more
 complicated with the naked molecule. The dummy atoms have no basis functions
 or any physical meaning, they just define additional (potentially useful)
 geometric variables as a mathematical tool.
 'Ghost atoms' are often used for Basis Set Superposition Error (BSSE)
 corrections, where you have to place basis functions in space - their
 position is defined by ghost atoms. They don't have a charge and hence don't
 change the physics of the system, BUT since they provide basis functions
 they change the quantum mechanical description of the system and hence the
 computed results. (Don't mix this up with external point charges from e.g.
 QMMM!)
 Again, sometimes these terms are used the other way round.
 Hope that helps. Best
 Johannes
 ---------------------------------------------------------------
 Johannes Hachmann
 (Dipl.-Chem., M.Sc.)
 Chan Research Group
 140D Baker Laboratory
 Department of Chemistry and Chemical Biology
 Cornell University
 Ithaca, NY 14853-1301
 USA
 ---------------------------------------------------------------
 > -----Original Message-----
 > From: owner-chemistry+jh388==cornell.edu-x-ccl.net
 > [mailto:owner-chemistry+jh388==cornell.edu-x-ccl.net] On Behalf
 > Of Antonio G. De Crisci antonio.decrisci##utoronto.ca
 > Sent: Friday, March 06, 2009 7:53 AM
 > To: Hachmann, Johannes
 > Subject: CCL: Banquo atoms Vs. Dummy atoms
 >
 >
 > Sent to CCL by: "Antonio G. De Crisci"
 > [antonio.decrisci[#]utoronto.ca] Hi all,
 >
 > Can someone tell me if ghost atoms (Bq) are as innocent as
 > dummy atoms (X)? Im aware that Bq atoms dont have basis
 > functions, but does this make it as innocent as dummy atoms?
 > If so, why would you need Bq atoms?
 >
 > Thanks,
 > Anthony
 >
 >
 >
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