Subject: Replies received: Improving ab initio computational speed

From: Laurence Lavelle <[email-masked]>
Date: Mon, 17 Nov 1997 13:53:22 -0800

Thanks for all the replies, they are listed below.
(It appears that there was a 4 day delay from the time I posted my message
to CCL.)

My original question was:
>Date: Sat, 08 Nov 1997 19:05:12 -0800
>To: [email-masked], [email-masked]
>From: Laurence Lavelle <[email-masked]>
>Subject: Improving ab initio computational speed
>
>Two possible ways to speed up ab initio calculations are:
>
>1) Use a bases set with fewer bases functions first. Then, after
convergence, use a bases set with more bases functions. Repeat this process
until convergence is obtained with the (largest) bases set that was
originally intended.
>
>2) Use the largest bases set with a higher 2e integral cuttoff point (e.g.
10^-7 Hartree) and after convergence decrease the 2e integral cuttoff
point. Repeat until convergence is obtained with the originally intended 2e
integral cuttoff point (e.g. 10^-11 Hartree).
>
>Do either of these approaches have potential downfalls. Is one approach
better than the other?
>Thanks
>Laurence Lavelle
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Date: Mon, 17 Nov 1997 14:03 -0600
From: "David Maxwell" <[email-masked]>
To: [email-masked]
Subject: Re: Improving ab initio computational speed

Dear Laurence,

Both of the methods you listed may be utilized to achieve your goal of
speeding
up the calculations.  When utilizing a lower basis set, be aware that there
may
not be a one-to-one correspondence of minima between different basis sets.
With
regards to the integral accuracy, I have only seen rare occasions where this
became a problem and it was usually when I was working with elements further
down on the periodic table.  Something you may also want to try is optimizing
with some geometrical values held fixed and then allowing them to relax in a
follow-up calculation.

Sincerely,
Dave

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Date: Fri, 14 Nov 97 09:47:22 -0330
From: [email-masked] (James Xidos)
To: [email-masked]
Subject: Geometry Optimisation

        The rate of convergence in a geometry optimisation is directly
dependent on the 
quality of your initial guess, and the quality of your Hessian (force
constant matrix).
The best route, through experience, of optimising structures at higher
basis sets is
to first, optimise at a lower basis set, followed by evaluating the Hessian
(force constant
matrix) at that lower basis set, then input both geometry and Hessian into
your higher basis
set job. Your second suggestion could help speed up the calculation of the
initial steps
in your higher level geometry optimisation, but a good geometry and Hessian
will eliminate
steps. I would be more inclined to increase the SCF convergence criteria
for the initial 
steps as opposed to manipulate the 2e integral cutoffs.
	James Xidos
	Memorial University of Newfoundland
	[email-masked]
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From: "Chris Murray" <[email-masked]>
Date: Mon, 17 Nov 1997 17:17:45 +0000
To: [email-masked]
Subject: Improving ab initio computational speed

Hi Laurence,

I'm surprised you got no responses since I always imagine there is someone
ready to answer this type of question.

1) Use a basis set with fewer bases functions first.
You can do this but it involve some algebra unless the large basis set is just
an augmented form of the first. I seem to remember the results are
disappointing in going from STO-3G to 3-21G say. It did not seem to be better
than forming a density matrix from a superposition of results from pre-stored
isotropic density matrices for atoms in the correct basis set. Good references
are:
Almlof's original Direct SCF paper or Cremer and Gauss, J. Comp. Chem. Vol. 7,
pp274 (1986).

2) My feeling about the 2e cutoff's is that it won't work well enough to
justify doing it. Also if you are doing a Direct SCF and using a change in
density matrix elements in the integral test then there is a danger of
inhibiting converence - in such cases it is advisable to increase the
tolerances slightly during the run.

Hope that this is some help.
chris
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From: "Stavrev, Krassimir" <[email-masked]>
To: "'Laurence Lavelle'" <[email-masked]>
Cc: "[email-masked]'" <[email-masked]>
Subject: 	RE: Improving ab initio computational speed
Date: 	Wed, 12 Nov 1997 14:44:22 -0500
Sender: [email-masked]
Reply-To: "Stavrev, Krassimir" <[email-masked]>

	Laurence:  The first approach makes more sense to me; the second
one may affect the accuracy of the calculations unpredictably. In any
case, with HyperChem 5 one can only export the eigenvector matrix and
reproduce properties when the matrix is imported back, but cannot use
the ext-file to form an initial guess of the Fock matrix.  Accordingly,
the SCF information that is stored at certain point is lost when one
proceeds with another single-point calculation. It would be nice to have
such read-from-file initial guess option  in the program; it would be
especially useful for people who run lengthy calculations and want to
restart the job from the point where the SCF procedure was terminated. 
	Regards,
	Krassimir  
	---
	Krassimir Stavrev, PhD, Director of Scientific Support
	Hypercube, Inc.  Florida Science  and  Technology Park
	1115  N.W.  4th Street  Gainesville,   Florida   32601
	Voice:(352) 371-7744/(800) 960-1871 Fax:(352) 371-3662

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Sender: [email-masked]
Date: Mon, 17 Nov 1997 09:03:08 +0100
From: Alexander J Turner <[email-masked]>
Organization: OCI - University of Zurich
To: Laurence Lavelle <[email-masked]>
Subject: Re: CCL:Improving ab initio computational speed
References: <[email-masked]>

Hi!

In reply to your question.  Both methods have merit, but there are
potential problems.

The main problem with the basis function idea is that it assumes that a
higher basis set calculation will converge faster from your low basis
set optimized structure than from your original guess.  This might well
not be the case.  It will tend to be the case much more if you compute
an exact hessian at the low basis set end point and use that as the
guess hessian for the higher basis set calculations.  This can be
especially so for HF -> MP2/DFT work.

Also, you assume that the lower basis set will be able to model your
compound at all.  STO3G can be so far from 6-31g* (say) that your
initial STO3G calculations 'lock' you into an unrealistic part of the
potential energy surface.  Such considerations are much more a problem
when you are looking for transition states.

The second method has a problem, it is virtually of no interest for
conventional SCF.  The time is taken computing the 2e integrals, not
converging the SCF.  For direct schemes, it is of interest, but only if
you are going to try and get a very tight convergence of the SCF.  I.E.
so tight you encounter slight convergence problems.  The normal property
of convergence is that the last bit takes less time than the initial
stages (because DIIS has more information to work with).  Thus you gain
little from the lower convergence, and that little you loose from the
extra optimization cycles.

So - my recommendation is inheritance of the structure _and_ hessian
>from a lower level of theory (or basis set) with extreme caution when
applied to saddle points.

Best wishes
Alex
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From: Eugene Fleischmann <[email-masked]>
Organization: 	Q-Chem, Inc.
To: Laurence Lavelle <[email-masked]>
Cc: [email-masked]
Subject: 	Re: Improving ab initio computational speed
Date: 	Thu, 13 Nov 1997 11:14:45 -0500

Unless your starting structure is really bad, I cannot recommend
approach 1) above, as it is more efficient to start with the final basis
set.  The optimized structure and hessian from a run with a smaller
basis set may be no better than a good initial guess at the initial
structure on the original structure.  Nevertheless, Q-Chem can handle
such projections of a smaller basis onto a larger one.  Regarding
approach 2) above, it is more efficient to adjust the integral cutoff
iteratively within the SCF scheme rather than stopping, reading, and
restarting the job using multiple single point calculations.  Such a
variable cutoff approach is already being used by a number of programs,
including Q-Chem.
				Best Wishes,
				Eugene Fleischmann
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