Constraints on an atom for optimization calculations


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First, the input you posted will not run for two reason. One, you have a typo in your DFT block; it should be b3lyp not b3ylp. Two, you are trying to apply a cartesian constraint to an optimization in internal coordinates, and the code can't currently handle that. To fix that you need to add noautoz to your geometry line.

Second, even with those fixes, the absolute coordinates of all your atoms will change initially because you have specified autosym on the geometry line. This will rotate and translate your molecule before anything else is done or any constraints are considered. If you want to retain the coordinates that you specified in the input file, change autosym to noautosym.