Subject: Re: CCL:What makes Gaussian fast ?

From: [email-masked] (Michael Frisch)
Date: Sat, 7 Jan 95 11:27:43 EST

Dear cyberchemists,
            I have posted this question a few days ago and I will summarize
    the respond and have them posted to the net very soon.
            I just would like to add some informations here as requested by
    some readers,
    
            - The review article which I read that Gaussian scale at N**2.7
    not at N**4 is
              Richard A. Friesner (1991) "New Methods for Calculations on
              Large Molecules". Annu.Rev.Phys.Chem. 42: 341-67.
    
            - My system is Li(I)6(NH3). Dunning's DZP bases are being used
    to perform Hartree-Fock calculation.
              The calculation is undertaken on IBM RS6000/550 and here is
    the result:
       G92                 30 minutes 59.2 seconds         E = -344.688161
       GAMESS     4 hours  48 minutes                      E = -344.6887325
            
            One can see that the Gaussian is much faster than GAMESS. I have
    tried running parallel GAMESS on a cluster and it is still slower than
    non-parallel Gaussian.
            The total energy is in Hartree unit. One can see that Gaussian
    speed up at the loss of accuracy but I am still satisfied because the
    interaction energy (comparative energy) from both programs remain the
    same (consistency).

When doing just a single-point SCF calculation, Gaussian 92 defaults to
doing the calculation to sufficient accuracy for the purposes for which
people normally do single-point calculations -- energy comparisons to a few
tenths of a kcal, potential-derived charges to a few digits, etc.  If you say
SCF=Tight on a single-point calculation, you will get the full convergence.
(Full convergence is the default for anything but a HF single-point
calculation -- HF optimizations, MP2 energies, etc.)  Full convergence is
about a factor of 2 slower than the cheap convergence.  The G92 (SCF=Tight)
and GAMESS energies should agree to 8 figures at least.

Mike Frisch
Gaussian, Inc.
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