direct vs. in core vs. disk i/o computations

Dave Moore dtmoore at
Fri Sep 6 07:58:24 BST 2002


Many people (including me), have asked this question in the past, and the
only solution I have ever found is the following (Quotation from my earlier
reply to another poster):

"Since MOLPRO does not seem to allow you to keep integrals in memory, even
when there is plenty of space, you have to trick MOLPRO into writing the
intergral table into memory by using a ramdisk.  Just in case you are
unfamiliar, a ramdisk is an area of memory that is mapped to have file
access like a disk drive ... sort of the inverse of virtual memory."

I have tried this approach to test a couple of simple examples, and it
seemed to work quite well (I actually used two ramdisks -- one each for
FILE's 1 and 4, but I don't this this is really necessary).  However, I did
not have a very large memory to map at the time, so I was not able to do
extensive testing.  Perhaps someone with more experience could chime in

	Also, one caution is that I think you would need to be quite careful that
the space on the ramdisk was never exhausted ... hopefully this would result
only in you MOLPRO job crashing like with a normal disk ... but in theory
you could overwrite some other area of memory, causing more serious

Dave Moore
FOM Rijnhuizen
Edisonbaan 14
3439 MN, Nieuwegein
The Netherlands
+31+30 609-6826
FAX:   603-1204

-----Original Message-----
From: owner-molpro-user at
[mailto:owner-molpro-user at]On Behalf Of Ludger Palm
Sent: Thursday, September 05, 2002 4:48 PM
To: molpro-user at
Subject: direct vs. in core vs. disk i/o computations

Dear Molpro community,

I understand that Molpro may store the two-electron integrals on
disk or that it may do a "direct" computation by computing these
integrals again in every SCF/MCSCF computation.

On a machine with big memory, I might wish to compute these integrals
only once, but not write them to disk. Instead I could and would like
to keep all the integrals in memory.

Can this be done with Molpro 2002.3 on an IBM Power4 computer?
If yes, how is it done?

With best regards
Mit freundlichen Gruessen

Ludger Palm

 Dr. Ludger Palm                           | Phone: +(0)89-289-28792
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