Subject: SUMMARY: Optimum memory allocation in GAUSSIAN94

From: "C.F. Matta" <[email-masked]>
Date: Thu, 22 Jun 2000 01:12:58 -0400 (EDT)

Hello all,

Three weeks ago I posted a question about RAM allocation and performance
of GUAUSSIAN94.  I'd like to thank those who responded:
	Dr. Douglas J. Fox   and   Dr. Daniel Glossman

Cherif

ORIGINAL QUESTION:
==================
In GAUSSIAN94, I noticed that the job CPU time as recorded in the log file
increases when I allocate too much memory (instead of decreasing).
For example, a geometry optimization of benzene at the restricted
Hartree-Fock level using a 3-21G basis set on a LINUX 5.2 platform
with a PENTIUM III 600MHz processor (768MBs RAM), took the following
times:
        200 MB ----> 1 minutes 18.5 seconds
        600 MB ----> 2 minutes 11.6 seconds
I noticed the same with other systems as well, larger memory=slower
job.  There must be a way to estimate the optimum memory allocation for a
specific job.
----------------------------------------------------------------------
Answer 1:
=========
Dr. Douglas J. Fox:  <[email-masked]>
===================
Cherif,
    While in principle the answer is yes it is not generally practical.
The variables include the number of atoms, number of basis functions
degree of contraction, maximum angular momentum, and method and generally
will differ from the SCF to the gradient to the second derivative.
    The default for G94 is 32MB which is sufficient for most HF and
DFT calculations.  There is modest cost when integrals require multiple
batches and additional memory will help but most calculations you are
willing to wait for will peak less than 256MB and any additional memory
will be wasted and only increase the overhead.  G98 increases the
default to 48MB but mostly for MP2 calculations.
   You are well advised to check chapter 4 for both discussion of
options for various methods and the table showing minimum memory for
various method/basis set combinations but generally setting a more modest
default and increasing it for calculations which need it will give you
good performance.

  Douglas J. Fox
  Technical Support
  Gaussian, Inc.
  [email-masked]
--------------------------------------------------------------------
Anwer 2:
========
Dr. Daniel Glossman: <[email-masked]>
====================

Dear Cherif:
I am using G98W and I have noticed the same behavior. Please share the
answers you receive.
                                                               Daniel
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___________________________________________________________________________

  Cherif F. Matta		    	  tel. (905) 525-9140 ext. 22502
  Chemistry Department                    fax  (905) 522-2509
  McMaster University                               _______________________ 
  Hamilton, Ontario, CANADA L8S 4M1.                | "Choice not chance
....................................................|  determines one's  
 Member of the Board of Governors of the University |  destiny". Anonymous 
___________________________________________________________________________