Subject: Re: CCL:counterpoise with LMP2

From: Gerd Raether <[email-masked]>
Date: Tue, 14 May 2002 14:28:14 +0200

Hi Tanja,

maybe the work of Istvan Mayer et al.

  author = 	 {I. Mayer and \'A. Vib\'ok and G. Hal\'asz and P. Valiron},
  title = 	 {A BSSE-Free SCF Algorithm for Intermolecular
		  Interactions. III. Generarlization for Three-Body
		  Systems and for Using Bond Functions},
  journal = 	 {Int. J. Quantum Chem.},
  year = 	 {1996},
  volume =	 {57},
  pages =	 {1049}

(see also the references there) is useful for you. This approach is
especially useful if you are going to use small basis sets.  Here you
also get a BSSE free wave function. Furthermore as far as I know you
will find also a discussion how the classical Boys Bernardi Method
should be applied - this should be sufficient if you just go for BSSE
free energies.

Gerd

   From: uccatvm <[email-masked]>
   Date: Mon, 13 May 2002 21:56:43 +0100 (BST)

   Hi all,

   I am wondering what the most correct way is to do counterpoise with local 
   MP2. In the local MP2 method originally proposed by Pulay (Chem.Phys.Lett. 
   100, 151, 1983), to each localised MO a subset (orbital domain) of the 
   virtual orbitals is assigned.  To calculate the interaction energy of a 
   weakly interacting system, the orbital domains of the subsystems are first 
   determined at large distance, and used in subsequent dimer calculations at 
   smaller intermolecular distances (as recommended in for example Schutz et al., 
   J. Phys. Chem. 102, 5997, 1998).

   Now, I assume that (to keep a true counterpoise) it is best to use the 
   orbital domains determined at large distance for the monomer+ghost 
   calculation. For this, one would first have to determine the domains of the 
   monomer+ghost with the ghost at large R, and use the thus obtained orbital 
   domains in the monomer+ghost calculation at the smaller distance. What 
   do people think? Would this be the correct way of doing it?

   Of course, the BSSE is strongly reduced in LMP2, and should in principle
   be negligible when using an appropriate basis set. However, when using 
   small basis sets it may not be negligible, and I would like to know the 
   best way of doing counterpoise for these cases.

   Thanks in advance,

   Tanja

   -- 
     =====================================================================
	Tanja van Mourik                                                
	Royal Society University Research Fellow
	Chemistry Department 
	University College London    phone:    +44 (0)20-7679-4663      
	20 Gordon Street             e-mail:   work: [email-masked] 
	London WC1H 0AJ, UK                    home: [email-masked]     

	http://www.chem.ucl.ac.uk/people/vanmourik/index.html
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From: uccatvm <[email-masked]>
Date: Tue, 14 May 2002 22:59:30 +0100 (BST)

Hi Gerd,

Thank you for your answer. However, my question was more specifically how
to get the best orbital domain match for the dimer and monomer+ghost
calculations in local MP2, and not so much how to do counterpoise 
calculations in general. 

The SCF-MI methods of Mayer and co-workers are a different approach to
deal with BSSE, and are not without problems.  For example, with 
SCF-MI/VB method and the aug-cc-pV5Z basis set, the De of He2 is 10.64 K 
at 5.8 bohr - J. Mol. Struct. (Theochem) 549, 77, 2001. The FCI/aug-cc-pV5Z 
estimate is 10.17 K at 5.6 a.u. (J. Chem. Phys. 111, 9248, 1999).  I 
think this shows that SCF-MI overestimates the interaction energy.

Tanja