Sep 27, 2007 at 12:01:32PM -0700, [EMAIL PROTECTED] wrote: > > Gaussian and GAMESS algs. typically do not store integrals unless they fit > into memory. It has been more efficient to just recompute them than to store > them.
For some reason my previous email did not make it through the list yet, but I will keep answering this thread, wishing my posts will eventually surface. Richard, You are only right about a SCF calculation. This method of recomputing (ERI) integrals in each SCF step is called Direct SCF by the quantum chemistry community. However, Andrew (IIRC the orginal poster) told us he will be running multi-reference jobs. It is different business here because you will be carrying some sort of Configuration Interaction (CI) and, even though there is also a direct CI method, you do have to calculate ERI integrals, transform and store them. This transformation will fit in core only for the very small problems, and usually you already need some decent disk-IO in this step. Actually depending on the size and details of calculation, matrices become so large that you need 100s of Gb, so fast disk-IO is a must. > So if you were working on smaller systems at modest levels of theory you > could put all the integrals in core and bandwidth would be very important. The smaller systems/problems that would fit in core (and finish in a matter of seconds) would probably take minutes to finish doing a direct (or semi-direct) method (recomputing), so no one needs a fancy workstation for them nowadays. > If you were working with larger systems and higher levels of theory you > probably would have to resort to recomuting the integrals and therefore would > be less concerned about bandwidth and more focused on clock and pure floating > point. Your probably job mix and benchmarking it will be key in making the > right choice. Remember that scalling with most QC methods is very steep (from O(N**3) to O(N**7))! So, depending on the size and level, even direct computations will not sufice and disk-IO will be very important as well. G -- Guilherme Menegon Arantes, PhD São Paulo, Brasil ______________________________________________________ _______________________________________________ Beowulf mailing list, Beowulf@beowulf.org To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
