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

From: Victor Lua~na <[email-masked]>
Date: Fri, 21 May 1999 13:43:23 +0200 (METDST)

> Date: Thu, 20 May 1999 17:35:09 +0000
> From: Richard Wood <[email-masked]>
>
> But 0.001/1  corresponds to 0.1%.  So any propagation of this gives maybe 
> 0.2%.  Is this important?  Probably not.
>
> On Thu, 20 May 1999, Laurence Lavelle wrote:
> > 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.
> > 
> > At 01:07 PM 5/20/99 +0000, Richard Wood wrote:
> > >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.
> > >
> > >On Thu, 20 May 1999 [email-masked] wrote:
> > >> >> 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?

   The point is not that calculations are more or less accurate. Nobody
disputes that you should report results after carefully considering its
significance. The point is that the output of a program, or the files
designed to interface several programs together SHOULD provide REPRODUCIBLE
results. Thats one of the essencial points of science. By producing a
gross round-off many programs preclude reproducibility or further usage
of their data.

           Regards,
                    Victor Lua~na

--
 HomePage  %%http://www3.uniovi.es/~quimica.fisica/qcg/vlc/luana.html%%
+----------------------------------------------+  +---^---/    /
!               Victor Lua~na                  !  |   ~       / Just in case
! Departamento de Quimica Fisica y Analitica   !  |           | you don't
! Universidad de Oviedo, 33006-Oviedo, Spain   ! <            / remember
! e-mail: [email-masked]%% !  |          /  where Oviedo
! phone: (34)-8-5103491  fax: (34)-8-5103125   !  |____  ___/   is  ;-)
+----------------------------------------------+       \/

---

From: Jochen Kuepper <[email-masked]>
Date: Mon, 27 Jun 2135 21:30:41 +0200

>gross round-off many programs preclude reproducibility or further usage

Well, if your number is accurate to some figure, you don't need to specify all
this ten numbers you calculated _behind_ it, since all results _are_
reproducible within their error by the given numbers.
(You probably need one more or so, but you get the idea here.)

Greetings,
Jochen
-----------------------------------------------------------------------
  Jochen Küpper

  Heinrich-Heine-Universität Düsseldorf     [email-masked]
  Institut für Physikalische Chemie I
  Universitätsstr. 1, Geb 26.43 Raum 02.29     phone ++49-211-8113681
  40225 Düsseldorf                             fax   ++49-211-8115195
  Germany             http://www-public.rz.uni-duesseldorf.de/~jochen
-----------------------------------------------------------------------

---

From: Brian Salter-Duke <[email-masked]>
Date: Sat, 22 May 1999 09:57:11 +0930

> (You probably need one more or so, but you get the idea here.)
There are two meanings to significant here and I think we are getting them 
confused. One is the agreement with experiment which is often very low. The
other is the number of figures that are significant for the model. Let
us take ab initio. We decide on a method say MP2, and we decide on a
basis set, say 6-311G(d,p). If we do this calculation on different machines 
and/or with different code we expect to get the same answer, but of course
we do not. We get result that are the same to maybe 12 figures if the
geometries are identical. If the geometries have been rounded we get much 
poorer agreement and we can not check whether one code is really doing the 
same as another code. We often need to make these checks. No code should 
round input data unless of course it contains more figures than the machine 
can handle.

Cheers, Brian.

-- 
        Associate Professor Brian Salter-Duke (Brian Duke)
School of Mathematical and Physical Sciences, Northern Territory University,
          Darwin, NT 0909, Australia.  Phone (61) (0)8-89466702
e-mail: [email-masked]  or [email-masked]