From b852b308ddb4d3d9bb6345ffdfbe89a0167d8bef Mon Sep 17 00:00:00 2001 From: Kurt Glaesmann Date: Wed, 10 Nov 2010 16:56:11 +0000 Subject: [PATCH] Add Huub's discussion to the SVN --- QA/tests/dft_bsse/README | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) create mode 100644 QA/tests/dft_bsse/README diff --git a/QA/tests/dft_bsse/README b/QA/tests/dft_bsse/README new file mode 100644 index 0000000000..451e456da6 --- /dev/null +++ b/QA/tests/dft_bsse/README @@ -0,0 +1,30 @@ +========= 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. +