Subject: Z matrix to cartesian coord. Question

From: "Charles G. James" <[email-masked]>
Date: Thu, 08 Dec 1994 14:27:17 -0500 (EST)

I do not mean this as an attack upon anyone or anything.  However this relates
to information I have to pass on to undergraduates in reference to
computational chemistry.  Just this week someone on this mailing list asked for
a way to get 10-15 (significant?) digits, after the komma(decimal), Cartesian
coordinates for a Ih symmetry molecule from the z matrix of a Gaussian
92 program. Are we at the stage where that many significant digits are given by
any computational chemistry method?  Are we at the stage where that many
significant digits are obtainable by any physical method used to measure
structure?  I am still warning undergraduates to beware of the number of digits
which can appear in calculations not connected to their measurements or their
original input.  Can I now say there are ways out there to get precision only
formerly dreamed by spectroscopist? Are there ways to get structural parameters
to this precision?

Charles G. James, Jr.           Assoc. Professor of Chemistry
Dept. of Chemistry,  University of North Carolina at Asheville
Asheville, NC 28804-3299, USA             Phone: (704)251-6443
FAX: (704)251-6041                     E-mail: [email-masked]

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From: [email-masked] (Bill Laidig)
Date: Thu, 8 Dec 94 17:15:10 -0500

On Dec 8,  2:27pm, Charles G. James wrote:
> Subject: CCL:Z matrix to cartesian coord. Question

Prof. James,

I believe you misunderstood the intent of the message you referred to. The
author wanted his high symmetry geometry to be converted from internal to
cartesian coordinates to 10-15 digit accuracy. The reason for this is not
that the structures are assumed to be good to this many decimal places, but
that programs that figure out symmetry from cartesian coordinates very often
need the symmetry exactly or nearly exactly before there are able to pick
it up. My interpretation is the standard 6 decimal places he has from his
g92 output file is not accurate enough for the program he is subsequently
using for it determine or to verify that the molecule really is Ih symmetry.

Bill

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From: [email-masked] (Gert von Helden)
Date: Thu, 8 Dec 1994 17:31:13 +22305458 (PST)

> using for it determine or to verify that the molecule really is Ih symmetry.

It seems to me that Gaussian92 determines symmetry by symmetry opperations
after which things have to be the same to within 10**-8. This default
behaviour can be changed by setting iops 17 and 18 in link 2 as 
Iop(2/17=n,18=n) where n is the 10**-n accuracy for symmetry determination.
However, I don't know how the rest of the code reacts to that.
Also, in link 2 one can tell the program to punch the coordinates
as I2,3E20.12 by setting iop(2/13=2). 
I did not try any of the above things but hope it helps.

Gert

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ Gert von Helden, Dept. of Chemistry, UCSB, Santa Barbara, CA 93106 +
+ Tel   : 805-893-2673, Fax   : 805-893-8703                         +
+ E-mail: [email-masked]                                     +
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