Gaussian is suddenly giving Imaginary Frequencies for stable molecules?

I'm calculating the energy of all the E isomers of octadecene using Opt+Freq with DFT B3LYP 3-21G. Everything was fine from 1-octadecene to E-5-octadecene, but from E-6-octadecene onwards it...

29 March 2023 4,757 4 View