Subject: Re: CCL:Significant figures and single point calculations

From: [email-masked]
Date: Thu, 20 May 1999 11:19:39 -0700

>> Many software packages round off significant figures.

What do you mean by "significant"? Do you believe the results of your
calculations (with all the approximations that go in there), or any experimental
data, is that accurate?

Btw, does anyone know what the uncertainty of position of a nucleus, say of a C
atom in a covalent bond, is due to thermal motion?

Wolfgang Huber
IBM Almaden Research Center
San Jose CA

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From: Richard Wood <[email-masked]>
Date: Thu, 20 May 1999 13:07:05 +36000

Based on my experience, which is limited, in calculations derived from MD 
simulations, that only the first place afer the decimal is significant.  
I other words, if a calculation gives a bond length of 1.510867 Angstroms, 
you should report it as 1.51 Angstroms.

Richard

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From: Richard Wood <[email-masked]>
Date: Thu, 20 May 1999 13:49:41 +36000

Make that 1.5 Angstroms, and not 1.51 Angstroms.

Richard

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From: Peter Shenkin <[email-masked]>
Date: Thu, 20 May 1999 16:17:20 -0400 (EDT)

Come to think of it, make that 1 Angstrom -- or actually, do
I mean 2?

Wait a second -- I think I see the problem.  :-)

	-P.

--
********* Peter S. Shenkin; Schrodinger, Inc.; (201)433-2014 x111 *********
*********** [email-masked]; http://www.schrodinger.com ***********

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From: Laurence Lavelle <[email-masked]>
Date: Thu, 20 May 1999 13:19:04 -0700

My point is self consistency in calculating single point energies that will
differ due to file format changes on what should be the identical file.

No the calculations are not accurate to 3sf in an absolute sense. But given
that much of computational chemistry involves relative energies, we should
at least get it right for two identical calculations done on the "same"
file/structure.

By the way, high resolution structures are determined to 3sf. Very high (V
low temp) small molecules structures can be determined to 4sf.

Laurence

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From: Laurence Lavelle <[email-masked]>
Date: Thu, 20 May 1999 13:26:26 -0700

Agreed. 

In fact my original posting makes it clear that this is a moot point for MD
and ab initio simulations because the change in atomic positions > 3sf. 

But single point calculations on what should be the identical file will
give different answers when bond lengths change by +/- 0.001A.

Laurence

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From: Richard Wood <[email-masked]>
Date: Thu, 20 May 1999 17:35:09 +36000

But 0.001/1  corresponds to 0.1%.  So any propagation of this gives maybe 
0.2%.  Is this important?  Probably not.

Richard