imaginary frequencies before the translations and rotations are projected out

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Forum Regular

7:02:11 AM PST - Tue, Feb 14th 2017
The short answer is that you probably want to re-optimize the geometry and re-calculate the normal modes with tighter thresholds (you shouldn't have to re-evaluate your functional/basis set choices unless you have other reasons for doing so). For a little more discussion on the matter, I recommend the following white paper, http://gaussian.com/vib/