# Re: direct vs. in-core calculations

*From*: rbw at.at
msc.edu (Richard Walsh)
*Subject*: Re: direct vs. in-core calculations
*Date*: Mon, 26 Apr 93 09:16:01 -0500

Doug,
If the memory is free and sufficient for the problem, you should
always use INCORE (although I am not sure if this option is available
on non-CRAY versions of the code). For closed shell systems memory
requirements scale as N^4/4 where N is the number of basis functions.
If you pay extra for memory then you need to consider the performance
boost you get from running INCORE. On a CRAY-2 it can be as much as
a factor of four. On the C90 it appears that it is a factor of three.
At some point the DIRECT algorithm overcomes the INCORE algorithm,
but this seems to be far beyond the basis function count that can
be fit INCORE on todays machines reasonably ... some 'memorable' data
points on the basis function memory relationship for closed shell
systems are 100 basis function or fewer fit inside 100+ MWs (64 bit
words, 200 basis needs 200 MWs (64 bit words), 300 basis needs 1+ GWs
(again 64 bit words). Open shell systems need twice as much space.
Sincerely,
Richard Walsh
Minnesota Supercomputer Center