Z-matrix,summary

 Dear netters,
 Some days ago, I submitted the following question:
 ************
 Just a simple question concerning geometry optimization using G94.
 If you want to keep the local symmetry of a functional group, say,
 a methyl group, how do you do it? Can one use algebraic expressions
 in the Z-matrix, such as in the following input for acetaldehyde:
 	H
 	C   1   1.08
 	O   2   1.33   1   120.
 	C   2   1.54   3   120.   1    180.
 	H   4   ch     2   hcc    1    hcch
 	H   4   ch     2   hcc    1    hcch+120.
 	H   4   ch     2   hcc    1    hcch-120.
 	ch  1.01
 	hcc  120.
 	hcch  180.
 *****************
 I have obtained several replies from kind people. The solution is
 quite simple, so I am a little ashamed. Nevertheless, it is good
 to know that G94 does not accept composite expressions.
 As an example, I reproduce below the reply from Stefan.
 ********************
 Hi JirKa,
 in G94 you have to use popt (or addredundant), if you want to constrain some
 degrees of freedom. It implies opt=z-matrix instead of the normal redefinition
 of your structure into redundant internal coordinates.
 You must not use algebraic expressions like "hcch+120" in the
 z-matrix, but you can achieve the same effect with the following z-matrix:
  H
  C   1   1.08
  O   2   1.33   1   120.
  C   2   1.54   3   120.   1    180.
  H   4   ch     2   hcc    1    hcch
  H   4   ch     2   hcc    5    120.
  H   4   ch     2   hcc    5    -120.
  ch  1.01
  hcc  120.
  hcch  180.
 Yours,
          Stefan
 Stefan Fau,               fau : at : mailer.uni-marburg.de
 FB Chemie der Philipps-Universitaet Marburg,
 Hans-Meerwein-Str.
 D-35032 Marburg
 ***************************
 Jirka Kozelka
 Universite Rene Descartes, Paris
 kozelka : at : citi2.fr
 fax +331-42 86 83 87