Subject: Gaussian94 Advantages vs. Gaussian 92

From: <[email-masked]>
Date: Thu, 1 Jun 1995 20:43:00 -0400

Since questions regarding Gaussian have resurfaced I thought this 
commentary might be helful.

We carry out Gaussian 92 and now 94 MP2/6-31G(d) calculations regularly and 
have run into most of the problems many CCL participants have been raising. 
Because of resource demands and our desire to work on more realistic 
systems, we restrict higher level Gaussian calculations to more robust 
systems:

     SGI Power Challenge 4xR8000, 512 MB RAM, 16 GB scratch space
     SGI Power Indigo2 R8000, 128 MB RAM, 4 GB scratch space
     SGI Power Indigo2 R8000, 96 MB RAM, 2 GB scratch space

And I have run calculations up to 128 basis functions restricted to as 
little as 2 MB with differing degrees of parallelism on the Power Challenge 
to evaluate factors impacting run times.  Gaussian 92/94 will try to select 
the optimum procedures within the specified memory limitation by trading 
off RAM and scratch storage.  But there are absolute minimum requirements 
before you start to swap, at which point your system grinds to a crawl, or 
you run out of disk space and the job crashes.

Keep in mind that the estimates offered are rough approximations based on 
information in the "Gaussian 92 User's Guide".  Storage estimates are in 
good agreement with my experience in using Gaussian 92.  Gaussian 94 may 
have implemented some storage reductions but I haven't noticed dramatic 
reductions in the past 1-2 months and have not conducted direct 
comparisons.

Memory
------

In principle, the approximate minimum free memory requirements are:

     Basis Functions  Units  100  150  200  250  300
     ----------------------------------------------- 
     SCF In-core, HF   GB    0.1  0.5    2    4    8
           UHF/ROHF    GB    0.2  1.0  3.2    8   16
     MP2 Direct, RAM   MB     2    6    16   32   64
     MP2 Semi-direct   MB     3    9    28   48   80
     ----------------------------------------------- 

Direct SCF calculations require even less direct MP2.  While 16-32 MB of 
RAM beyond the system needs is adequate for most jobs, we find that 
additional memory about double the minimum offers a 10-100% speedup in CPU 
time and helps minimize I/O.

Scratch Space
-------------

Generally, scratch storage is more critical than memory, especially for MP2 
calculations, in determining the absolute job size limit.  To minimize 
scratch files, you can try the following options which successively reduce 
your job limit within the 2.0 GB single file size and 8-16 GB file system 
size limitation of many popular workstation operating systems:

     Options                                         Approximate Maximum 
                                                  Number of Basis Functions
                                                     Within 2.0 GB Limit
     ---------------------------------------------------------------------
     SCF=Direct                                               220
     ...MP2 [single point]                                    220
     ...Freq                                                  180
     ...MP2=Direct Opt=CalcHFFC                               180
     ...MP2=(Direct,Stingy) Opt=CalcHFFC Freq=NoRaman         165
     ...MP2=(Direct,Stingy) Opt=CalcHFFC Freq                 150
     ...MP2=(Direct) Freq                                     135
     ---------------------------------------------------------------------

Currently, DEC along with Cray, Convex and possibly others I am less 
familiar with, offer UNIX operating systems which do not have these file 
and file system size limitation.  Presumably SGI, IBM, HP, SUN and other 
UNIX system vendors are working on implementing extended file systems which 
will overcome these restrictions but user demand has obviously gotten ahead 
of the suppliers.  In response, Gaussian 94 has implemented user definable 
temporary 'rwf', 'int' and 'd2e' scratch file names.  In addition, 'rwf' 
output, usually the largest, may be parsed over multiple files.  This 
capability greatly increases the job size that may be tackled by allowing 
you to more effectively distribute output over multiple files and multiple 
file systems but the amount of output, to my knowledge, is about the same.  
You still need the free disk space!  So, to look at the problem another 
way:

                    Approximate Disk Space Required (MB)

     Basis          - SCF -      -------- MP2 ---------
     Functions        Freq.      Direct    Freq. STINGY
     ---------      -------      ----------------------
         60             26          12           45
         80             82          36          145
     ---------      -------      ----------------------
        100            200          90          350
        120            415         185          725
        140            770         340         1350
        160           1300         580         2300
        180           2100        1000         3700
     ---------      -------      ----------------------
        200           3200        1400         5600
        220           4700        2100         8200
        240           6600        2900        12000
        260           9000        4100        16000
        280          12000        5500        22000
        300          16000        7200        28400
     ---------      -------      ----------------------

Direct SCF storage requirements are similar to direct MP2.  

Gaussian 94 vs. 92
------------------

While there may be some reduction in the scratch output, you will probably 
still have to contend with the scratch space requirements.  However, 
Gaussian 94 allows you to distribute the load over multiple files and 
multiple file systems.  So stock up on FAST disks (see commentary below).  
They're still much cheaper than memory or more CPU's.

There is some CPU speedup particularly for paralell SCF and parallel MP2.  
No, Gaussian 94 isn't 2x faster than Gausisan 92 but I have gotten 20% on 
some calculations.

Gaussian 94 implements several new features.  The contiuum reaction-fields 
for SCF and DFT optimizations (MP2 energies only) may be of interest and 
there are significant improvements to the built-in basis sets.

Lastly, Gaussian 94 installation and compilation did procede run much more 
smoothly than the last Gaussin 92 release.

Commentary
----------

The large scratch file space requirements of Gaussin 92/94 for larger jobs 
easily makes I/O a significant portion of the total run time particyularly 
with the newer faster processors.  Combined with the memory demand, I/O 
performance can significantly impact the throughput.  For this reason, it 
is essential, in most sitiuations, that the scratch file space be local - 
NFS over standard Ethernet is a killer!  

Experience with our Power Challenge running a large number MP2 
optimizations and frequency calculations generating 1-2 GB of scratch files 
helps to illustrate the problem.  An external striped array of single-ended 
fast-wide SCSI-2 disks on a single SCSI-channel limited data transfer to 10 
MBs with actual rates of about 8 MBs.  Analysis of run times with this 
configuration suggested that I/O accounted for 30-50% of the total run 
times.  Implementation of dual striped internal differential fast-wide 
SCSI-2 scratch file system increased transfer rates to 16-18 MBs and 
decraesed I/O to 10-20% of the total run time.  Recent installation of an 
external dual-channel differential fast-wide 4 disk array boosted transfer 
rates to about 36 MBs with I/O now consistently <<10% of the total run 
time.  The latest I/O configuration ensures efficient performance in 
tackling larger problems.  Note that such heavy I/O easily swamps caches, 
so RAID controllers with supplemental memory offer no benefits.

I/O requirements will grow exponentially with the problem size.  Therefore 
others can similarly expect to find that scratch storage and data transfer 
rates may be limiting factors in dealing with larger problems especially 
with PC systems, either under Windows or Linux (no, I don't know of a 
port).  Users would be well advised to consider systems offering fast-wide 
differential SCSI interfaces and/or multiple channel I/O capabilities for 
tackling more substatntial calculations.  Other more expensive, 
high-performance options such as HIPPI are also available but generally 
unwarranted for desktop systems.  The prognosis for the near future is 
actually rosey.  HP has announced a new disk controller capable of 
supporting >=100 MBs.  So, with a bit of luck, improved I/O options may 
appear on the desktop in time to match the next generation of UNIX 
processors and improved operating systems which support unlimited file 
sizes.

Hope this helps!

For specific information, try contacting Gaussian Inc. directly through 
[email-masked].  Also, the "Gaussian 94 Users's reference" may prove 
very helpful.

Karl
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| Comments are those of the author and not Unilever Research U. S.      |
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| Karl F. Moschner, Ph. D.                                              | 
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