Hello, I have a very old python script that reads through a frequency calculation output file (from earlier versions of Psi4 like 1.1) and returns the normal modes. I needed to use it again so I was updating it for the newer output format (particularly the Psi4 1.5 output format). I believe 1.1 was printing out the unnormalized Cartesian eigenvectors of the Hessian, and I can see from `psi4.driver.qcdb.vib.print_vibs`

that Psi4 1.5 prints the normalized Cartesian eigenvectors. What I can’t figure out is why the sign of some of the eigenvectors is the same from both codes, but the sign is flipped for other eigenvectors. I show output files from both Psi4 1.1 and Psi4 1.5 below for the same molecule/geometry. The sign is the same for eigenvectors (listed in printed order) 1, 3, 4, 5, 7, 9, 10, 13 and 15, while it is flipped for 2, 6, 8, 11, 12 and 14. My thought was that the sign convention should be either flipped or remain the same for all eigenvectors, but my knowledge of normal modes is pretty rudimentary. Is there something obvious I’m missing?

freq_psi41.1_acetate.txt (591.0 KB)

freq_psi41.5_acetate.txt (620.2 KB)

The old version of Psi4 1.1 I downloaded to test this is from http://vergil.chemistry.gatech.edu/psicode-download/1.1.html

and the version of Psi4 1.5 is the current stable release from

https://psicode.org/installs/v15/

Thanks for any potential help.