Hello Everyone,

I am doing some anharmonic vibrational calculations for small molecules with G16. I tested it with the VCD spectrum of (R)-propylene oxide. It was all good, finetuned the settings, got all real frequencies for harmonic and anharmonic vibrations. Also, experimental and calculated VCD spectra agreed very well.

Next, a bit bigger molecule. However, I got all real frequencies for the harmonic vibrations, but got imaginary frequencies with the anharmonic treatment for the first two fundamental vibrations plus a few combinations and overtones of these. I recalculated with stricter convergence criteria and finer grids, but didn't change anything significantly. The most important keywords with my latest calculations (but no improvement):

"opt=(calcall,verytight) freq=(vcd,anharmonic) ... empiricaldispersion=gd3bj scrf=(iefpcm,solvent=chloroform) nosymm int=superfine"

What could be done to get all real anharmonic frequencies?

Best regards