[gpaw-users] Imaginary frequencies in vibrational analysis

[Jens Jørgen Mortensen]

On 09/23/2017 04:42 PM, Divya Bohra - TNW via gpaw-users wrote:
>
> Dear All,
>
> I am performing thermochemistry calculations in the Harmonic limit 
> using GPAW for adsorbates on metal surfaces to calculate entropy and 
> enthalpy corrections to the electronic energy. Despite having a very 
> strict BFGS convergence criteria of 0.001 ev/Ang and perturbing the 
> starting geometry for the optimization a few times, I am still getting 
> the first vibrational frequency and thus energy as imaginary.
>
> My adsorbate has 5 atoms and below are the derived vib freq:
>
> (0.000000000000000000e+00+8.315037425202136490e+01j)
>
> (1.133595726253269760e+02+0.000000000000000000e+00j)
>
> (1.988078878927153426e+02+0.000000000000000000e+00j)
>
> (2.270980611466232233e+02+0.000000000000000000e+00j)
>
> (2.560765520188753612e+02+0.000000000000000000e+00j)
>
> (3.265759157205902738e+02+0.000000000000000000e+00j)
>
> (5.463490962450439383e+02+0.000000000000000000e+00j)
>
> (7.730608126631944970e+02+0.000000000000000000e+00j)
>
> (1.066735330313886607e+03+0.000000000000000000e+00j)
>
> (1.093583314262479007e+03+0.000000000000000000e+00j)
>
> (1.217372382427615548e+03+0.000000000000000000e+00j)
>
> (1.358510564043398972e+03+0.000000000000000000e+00j)
>
> (1.462720304851941137e+03+0.000000000000000000e+00j)
>
> (2.761239970843860192e+03+0.000000000000000000e+00j)
>
> (2.872013016024760418e+03+0.000000000000000000e+00j)
>
> Can I simply ignore this first vibrational frequency and perform the 
> thermochemistry calculations for the remaining 3N-1 energies? From my 
> understanding, assuming it to be 0 will give an issue in the internal 
> energy and entropy calculations.
>
> Does this imaginary frequency suggest I am on a TS or is it simply a 
> numerical artifact?
>
> In some places, it is mentioned that the 3N-6 frequencies are from 
> vibration and the first 6 are from rotation and translation and can be 
> ignored. However, GPAW takes all the 3N energy values for the 
> thermochemistry calculations. Can someone please clarify this?
>
> Any further information about this would be very helpful! Thanks in 
> advance.
>

You could try to play with nfree=4 and different values for delta:

https://wiki.fysik.dtu.dk/ase/_modules/ase/vibrations/vibrations.html#Vibrations

Maybe do the playing around for a small test system.

Jens Jørgen

> Best regards,
>
> Divya
>
>
>
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