[molpro-user] quinone partial optimization

[Leon Freitag]

Hi George,

this is what I suspect happens here (someone please correct me if I'm wrong)

it looks like your z-matrix does not reflect the C2 symmetry. You have too 
many variables, e. g. different variables XC3 and XC5 for what would be the 
same variable in C2 symmetry. Once you rotate the molecule around the C₂ axis, 
although the bond length does not change, the variable XC3 becomes XC5. 
Initially, they have same values, but because optg is using the internal z-
matrix coordinates as optimisation coordinates (at least if you use 
coord=zmat), you end up having one degree of freedom more, i. e. if XC3 
becomes for some reason unequal to XC5 during an optimisation cycle, you no 
longer have a geometry constrained optimisation. Try building one half of your 
molecule and then the other symmetric half by using variables from the first 
half -- just don't use too few variables as you'll have too few DoF.

See also this post: http://www.molpro.net/pipermail/molpro-user/2008-
November/002726.html

Leon
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