memory and disk requirements

[H. -J. Werner]

Dear Molpro users,
recently, questions about memory and disk usage
in ccsd(t) came up again. It is difficult to make
estimates with symmetry. Without symmetry (nosym, c1),
the memory requirement of the (closed-shell) triples 
code can be estimated as

minmem = 3.5 N^3 + N^2 * (m^2+2m+2) words

where N is the number of virtual orbitals, and m the number 
of correlated orbitals. The CCSD without triples needs much
less (of the order of  N^2 * (m^2+m+10). More memory may
lead to less I/O and shorter elapsed times. 

On a machine with 2 GB of memory the limit for a ccsd(t) without
symmetry is about 400 basis functions (assuming 20 correlated orbitals).
This is perhaps more than what can be afforded in terms of CPU time.

The bottleneck in disk space is in the integral sort, since
on the sort file the integrals are not compressed and stored
with labels. The max disk space for the 2-electron integrals
is approximately (N basis functions, no symmetry):

N^4/4 for the sort file (file 4)
N^4/8 for the main integral file (file 1). 

In practice, the integrals on file 1 are compressed and much less 
is needed. On file 4 only the significant integrals are stored, and
the actual file length is often a factor of 2-4 shorter.
The minimum  memory requirement for the sort is about
N^4/1600.

A calculation with 350 basis functions (no symmetry) needs
tyically about 30 GB of disk space. I have done such calculations
on Linux 32-bit systems without problems.

Please also note:  

1.) The max memory Molpro uses is specified as, e.g., 

memory,150,MW

and NOT memory=150 MW (1 MW = 8 MByte). Such wrong input has lead to some
of the recently reported problems!!

2.) You should not specify more memory than your machine
has. This could lead to excessive paging. As a rule of thumb,
don't use more than (MEMMAX-200)/8 MW, where MEMMAX is the 
memory your machine as in Mbyte.

Hope this helps!
Joachim Werner








-- 
Prof. Hans-Joachim Werner
Institute for Theoretical Chemistry
University of Stuttgart
Pfaffenwaldring 55
D-70569 Stuttgart, Germany
Tel.: (0049) 711 / 685 4400
Fax.: (0049) 711 / 685 4442
e-mail: werner at theochem.uni-stuttgart.de