>I am trying to perform a geometry optimization while constraining one of the torsional angles in my molecular system. I'm using state-averaged CASSCF and I would like to optimize on the ground state potential energy surface. The problem I am facing is that during the optimization, the torsional angle that is intended to be fixed, changes significantly (from 110 to 128.6 degrees). I using the following commands to fix the desired torsional angle:
> nzvar=1 ! in $CONTRL
> $ZMAT DLC=.T. AUTO=.T. nonvdw(1)=1,26
>I would also like to mention that I used the same set of commands to optimize the same molecule at a different fixed angle on the excited state surface. The latter did not have this problem. The input and output files are attached.
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[ v90copt.inp ] input file