From chemistry-request@ccl.net Sat Mar 14 15:02:08 1992 Date: Sat, 14 Mar 92 12:21:11 EST From: bernhold@qtp.ufl.EDU Subject: Re: coupled cluster methods question To: "DOUGLAS A. SMITH" , chemistry@ccl.net, Status: R On Mar 13 5:37pm, "DOUGLAS A. SMITH" wrote: > 1. Is it the best method currently available for open shell molecules? On Mar 14 3:11am, Ole Swang wrote: > This seems a rather vague concept to me... [And goes on to make some > good points about zeroth-order descriptions, multireference character, > etc.] This is indeed very vague -- and very application dependent, so I won't try to answer it. I will mention, however, that CC methods, especially those including triple excitations, can handle many systems that might otherwise be considered multireference quite well. On Mar 14 3:11am, Ole Swang wrote: > (unfortunately, as far as I know nobody has implemented a general > multireference coupled cluster algorithm as yet). Multi-reference CC methods are presently a very active research in a number of groups. Work is progressing rapidly. On Mar 13 5:37pm, "DOUGLAS A. SMITH" wrote: > 2. How will CCD or CCSD(T) perform on closed shell molecules? I have > several systems to compare, and I would like to use a single > consistant method for my calculations. In general CC methods well perform quite well for closed shells, and if the study is done carefully, there is no reason why CC results for closed- and open- shell systems can't be compared. For a more detailed comparison, it is necessary to have a better idea of what you intend to study. There are numerous studies in the literature of the quality of CC methods. Your best bet is probably to look up some of these involving species related to your own interests. Note: CCD is rarely used these days -- CCSD is much better at minimal additional cost, particularly if you intend to treat open shells. On Mar 13 5:37pm, "DOUGLAS A. SMITH" wrote: > 3. Is there a minimum basis set that should be used for CCD or > CCSD(T)? Is there a practical limit to the size of a molecule > which can be looked at using these methods? On Mar 14 3:11am, Ole Swang wrote: > Generally, a double-zeta valence plus polarization basis set (like, > say 4-31g*) should be thought of as a minimal basis set for > calculations involving correlation (something like STO-3g for Hartree-Fock). The collected experience in our group is also that you need at least a polarized double-zeta quality basis. I would caution, however, that 4-31G* _doesn't_ qualify. From work we've done, even 6-31G* is marginal for any system that is not exceedingly well-behaved. Basically, the restriction of exponents use in the construction of the Pople-type basis sets means that even something with enough funcitons to be double-zeta really doesn't have the necessary flexibility. You're much better off with (for example) a nice standard Dunning DZP basis. For larger-than-DZP bases, and assuming you have a package that can handle it, generally contracted basis sets, such as Almlof & Taylor's ANO bases or Dunnings "correlation consistent" bases are a significant improvement over the more traditional segmented contraction sets. On Mar 14 3:11am, Ole Swang wrote: > There is, of course, a practical limit for the size of the systems which can > be studied using CC, or any other ab initio method. Where the limit goes > is depedent of the available resources. This is true for _any_ method, not just ab initio ones -- there is always a tradeoff, and there is not enough informaton provided to give meaningful answers. In our group, we generally study molecules that _aren't_ well-bahaved and require a high-level correlated treatment. We're accustomed to doing CC with triples corrections of various sorts for almost everything we work on. Given enough CPU and disk resources, it is possible to routinely calculate triples corrections for several hundred basis functions. There is no reason in principle why one can't go beyond that, but since triples costs scale roughly as n**7 (n basis functions) or higher, it can become quite expensive, and at some point you have to ask yourself if its really worth it (larger molecules are often _better_ behaved -- and therefore less in need of triples corrections -- than smaller ones). On Mar 14 3:11am, Ole Swang wrote: > in most cases a CC treatment is a factor 3 or 6 or something more > expensive than a CI treatment. (there shouldn't be an exponential > differennce betweeen the two). This is a fairly common misconception. For a CC calculation and a CI calculation with the same excitation operators included and the same underlying formalism, the cost is essentially the same. A difference you will see is that most CI codes are completely spin adapted, while most CC codes are written in a spin-orbital framework. The differences between a CC and CI implementation, then, depend on whether the spin-orbital CC has been implemented to also take advantage of the spin adaption when possible (for a closed shell reference). For an open-shell molecule, there are effectively several CC equations being solved simultaneously, to it's going to take longer than a spin-adapted CI code. On the other hand, CC methods are size-extensive (important for treating larger systems), and recover more of the correlation energy for a fixed amount of computational resources than CI. -- David Bernholdt bernhold@qtp.ufl.edu Quantum Theory Project bernhold@ufpine.bitnet University of Florida Gainesville, FL 32611 904/392 6365 From chemistry-request@ccl.net Sat Mar 14 15:14:54 1992 Date: Sat, 14 Mar 92 16:41:46 +0100 From: Carlos Lucasius Subject: Chemometrics in Canada To: chemistry@ccl.net Status: R Hello, I am interested in gaining some international research experience in the field of 'chemometrics'. Chemometrics is the information processing part of analytical chemistry and encompasses computational techniques such as multivariate statistical data analysis, calibration, experimental design, signal processing, pattern recognition, optimization, expert systems, neural networks, simulated annealing, etc. I have a clear picture about the chemometric world, except for... Canada. So my question is: does anyone know about Canadian research institutions or universities where chemometrics is practised? Many thanks for any useful pointer in advance! C.B. Lucasius Chemometrics Research Group Faculty of Science Catholic University of Nijmegen Toernooiveld 1 6525 ED N i j m e g e n T H E N E T H E R L A N D S E-mail: lucasius@sci.kun.nl From states@ncbi.nlm.nih.gov Sat Mar 14 17:32:40 1992 Date: Sat, 14 Mar 92 17:33:43 EST From: states@ncbi.nlm.nih.gov (David States) To: jkl@ccl.net Subject: Re: Time step in MD Status: R |> The discussion on the cutoff distances and time steps is very stimulating. |> However, I do not understand something here. It was not clear to me from your note what you did not understand, but it seemed to be related to Monte Carlo step size. The choice of step size in MC is purely an issue of computational efficiency. Too small a step and you spend all of your time sampling the same region of conformational space. Too large a step and you reject an excessive number of steps because they have unfavorable energies. I apologize if I have totally missed the point of your question. |> Jan Labanowski |> Ohio Supercomputer Center |> jkl@ccl.net David States National Center for Biotechnology Information / National Library of Medicine