From mforster()at()nibsc.ac.uk Tue Mar 17 11:18:26 1998 Received: from nibsc.ac.uk for mforster ( ( at ) ) nibsc.ac.uk by www.ccl.net (8.8.3/950822.1) id KAA25603; Tue, 17 Mar 1998 10:30:12 -0500 (EST) Received: from chalsig.nibsc.ac.uk by nibsc.ac.uk via ESMTP (951211.SGI.8.6.12.PATCH1502/940406.SGI.1(NIBSC)) for id PAA09315; Tue, 17 Mar 1998 15:30:00 GMT Received: by chalsig.nibsc.ac.uk (951211.SGI.8.6.12.PATCH1502/client-1.3.1(NIBSC)) id PAA05126; Tue, 17 Mar 1998 15:29:59 GMT Date: Tue, 17 Mar 1998 15:29:59 GMT From: mforster -x- at -x- nibsc.ac.uk (mforster) Message-Id: <199803171529.PAA05126 (+ at +) chalsig.nibsc.ac.uk> To: chemistry "at@at" www.ccl.net Subject: summary Dear Net Chemists Here is a summary of responses to my recent question concerning calculation of intermolecular interactions by electronic structure methods. There were no responses to the part of the query asking how the use of numerical basis sets, as opposed to contracted gaussians, would alter the effect of basis set superposition error. Here is the question itself. > Could some Quantum chemistry experts please offer some advice or > comments on the computation of intermolecular interactions by > quantum methods. > > I noticed a recent paper in which the the difference in the (SCF) > energy of an ion pair at some separation (r) relative to the the sum > of the energies of the individual ions was used to estimate the stabilisation > of the ion pair. Does this approach neglect basis set superposition effects > where the 'effective' basis set size grows as the two separate molecules > are brought together. In addition how would such calculations be affected > by numerical basis sets (eg as used by DMOL). Here are the responses: From: scheiner()at()chem.siu.edu (Steve Scheiner) intermolecular interactions in reply to your question on the net about interactions, your question is a good one, and one that has generated quite a bit of work over the years. the approach you mention may or many not have included a BSSE correction; you would have to read the details to know for sure. about the second part of your question, the basis set has a great deal to do with the result computed. so i can't answer your question except to refer you to the original paper. for more details, you might consult a book which i recently edited for wiley. the title of the book is Molecular Interactions. you would probably be most interested in chapters 3, 5 and 9. ----- Steve Scheiner, Professor scheiner(-(at)-)chem.siu.edu Dept. of Chemistry & Biochemistry Mail Stop 4409 Southern Illinois University Carbondale Illinois 62901 ph: 618/453-6476 fax: 618/453-6408 From: noertema ( ( at ) ) theochem.tu-muenchen.de (Folke Noertemann) Dear Dr Forster, since I am not really an expert, I may be telling you things that you are already aware of. Of course the effective basis, as you call it, grows if the distance between two molecular fragments is decreased. However, it is possible to correct the energies for the BSSE and so I think your first question cannot be answered without having seen the paper. Moreover the BSSE does not depend on the distance alone but also one the quality of the basis. It is for example possible to get a very low BSSE with a minimal basis-set calculation because for reasonble distances only the diffuse functions centered on fragment a can be used for the representation of orbitals on fragment b. Unfortunately I do not have any experience at all with numerical basis sets and thus I am unable to comment on your second question. I hope this is of some help to you, F. Noertemann Folke Noertemann Lehrstuhl f. Theoretische Chemie TU Muenchen Lichtenbergstr.4 85748 Garching noertema %-% at %-% theochem.tu-muenchen.de From: Richard Wheatley This is an extremely widely studied area of chemistry. Despite the many hundreds of postdoc-years and hundreds of postgrad-years spent on it, there is still no way of doing accurate, cheap calculations for any system with more than about 20 electrons or more than about 3 degrees of freedom. By accurate I mean guaranteed within about 10%, and by cheap I mean less than a few months of supercomputer time for the surface. Don't let anyone convince you otherwise! Having said that, SCF calculations will be (1) in error due to the incorrect polarizability of the negative ion. This is easy to look up (both SCF and correlated results are known). Also the (fairly small) dispersion interaction energy is neglected. If the ions are not spherical, SCF will also get the multipoles wrong and this will give major errors. (2) free of BSSE (reasonably!) provided that at least 10 - preferably more - Gaussian functions are used per spin-orbital, so 100 Gaussians would just be tolerable for F- and Na+, for example. The Counterpoise method can always be used to estimate BSSE if in doubt. I don't know about numerical basis sets. Note that BSSE gets MUCH WORSE, not better, for correlated calculations with the same basis set. For light reading, try B Jeziorski, S S Xantheas, A J Stone or possible some of the P W Fowler work. Or me. Best wishes Richard Wheatley, Department of Chemistry, University of Nottingham. Dr Mark J Forster Ph.D. Principal Scientist Informatics Laboratory National Institute for Biological Standards and Control Blanche Lane, South Mimms, Hertfordshire EN6 3QG, United Kingdom. Tel 01707 654753 FAX 01707 646730 E-mail mforster #*at*# nibsc.ac.uk