Subject: Cartesians -> Z-matrix ?!

From: <[email-masked]>

Dear Anna,

  I just read your request for a program which converts cartesian coordinates
into Z-matrices. Here is my comment:

  What you require is my no means trivial! Especially if you have symmetry, you
will want to devise your Z-matrix in a form which allows you to maintain this
symmetry (or relax it in a controlled fashion to a lower symmetry) and this
requires a degree of intelligence which would be hard to put into a computer
program. Secondly, even if you do not have symmetry, the number of possible
Z-matrices for a given set of cartesians is infinite, some of them being more
reasonable others less. Even if you had a program which could recognize the
more reasonable ones, there would still be a vast choice. Which of those is
suitable for you depends on your problem.

  I think there are two solutions to your problem (I also hate to spend time
concocting Z-matrices): (a) most current quantum chemistry programs allow you
to do optimizations in cartesian coordinates or even mixed cartesian and
internal coordinates (Gaussian). In many cases (rigid, polycyclic systems are a
good example) this is better anyway, and maintenance of symmetry is no problem
if you introduce cartesian variables and require that some of these must
remain identical during the optimization. (b) There are programs which let you
calculate any bondlengths, angles and dihederals from a set of cartesians (we
have one of those which works graphically, but we would need to interface it
to the format of your cartesian coordinate files) which may greatly facilitate
the task of setting up a Z-matrix if this is desired or needed.

  I hope that these comments are not too much of a disappointment. If you do
indeed find a program which converts cartesians into Z-matrices (realiably and
reasonably!), please let me know.

  thomas

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|  Prof. Thomas Bally                    |  E-mail:  BALLY@CFRUNI52          |
|  Institute for Physical Chemistry      |                                   |
|  University of Fribourg                |  Tel:     011-41-37 826 489       |
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