[gpaw-users] Imaginary frequencies in vibrational analysis

[Divya Bohra - TNW]

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.

Best regards,
Divya
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