TURBOMOLE Modules > Aoforce and Numforce

Stubborn imaginary frequency

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antti_karttunen:
Hi,

I'm not aware of any possibility to use 5 points instead of 3 in the numerical differentiation. The ruecker binary that generates the displacements only takes in $delta, $type (central/polyedr), $dimension (small, norm, big - this is a mystery to me!), and $freezeoption (nofreeze).

I agree that this sounds like a numerical artifact, the imaginary frequency is so small. From NumForce side, I only can think of two possibilities:
1) Using smaller or larger displacement (I guess you already tried these)
2) Using polyedr (if you so far used only central).

From ricc2 side, I don't know if increasing nexc could little bit affect the numerics. I guess you would see much larger imaginary frequency if this would be an issue of root flipping for some displacement.

I feel your pain, we are right now struggling with similar annoying small imaginary mode in phonon dispersion calculations for a solid-state material  ;).

Best,
Antti

uwe:
Hi,

just as a check: Distort the molecule along the vibrational mode with imaginary frequency and generate a series with different amplitudes (use TmoleX to do that or the vibration tool on the command line, run 'vibration -help' first to see the options and then use the -vibro option). Then, for each of the generated structures, run a single-point calculation and plot the energy (or excited state energy) vs the distortion. It will be very flat, but perhaps you can see whether it is a minimum or a maximum.

Regards,
Uwe

marand:
Thank you for a useful suggestion. I have done that and the PES scan says the molecule should be planar. although the surface is close to planarity in the vicinity of the supposed minimum.

However, I have also checked the aug-cc-pVTZ basis set and the addition of diffuse functions reduced the imaginary freqency for abou 50 to 7 cm-1.

I guess I will have to live with that.

My very best regards!
Marcin

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