Unstable modes in fitfc

Alloy Theoretic Automated Toolkit (ATAT)
1

I recently tried to calculate the vibrational entropy of BCC Au. After I optimized the structure (no inflection detection needed), I used

fitfc -ernn=4.0 -ns=1 -ms=0.02 -dr=0.1 -nrr

and for each folder I did

runstruct_vasp -lu -w vaspf.wrap srun

with KPPRA=8000 in vaspf.wrap. Then I did

fitfc -f -frnn=1.5 -kp=8000 -dr=0.1 -fu

and I had a lot of unstable modes in unstable.out. I picked the one numbered as 7 and did a “-gu”

fitfc -f -frnn=1.5 -kp=8000 -dr=0.1 -gu=7

to generate the unstable mode in folder “p_uns_0.1_8000_7”. I did another single point calculation in the folder with

runstruct_vasp -lu -w vaspf.wrap srun

and after the calculation,

fitfc -f -frnn=1.5 -kp=8000 -dr=0.1 -fu

I can see the calculation result of mode 7 is taken into account, with output like:

Reading…
vol_0
p+0.1_9.82073_0
p+0.1_9.82073_1
p+0.1_9.82073_2
p_uns_0.1_8000_7
Generating Fitting matrices
1/480
121/480
241/480
361/480
Performing least square fit…
Problem size= 480 x 78
Done
Creating Force constants…
Done
Performing normal modes calculations
kmesh= 20 20 20

Now I still have the mode 7 in the “unstable.out", which means the -fu and -gu process did not eliminate the unstable mode in this case. Did I make anything wrong? What can I do here?

2

It could be that, given the current range of force constants, it is not possible to reconcile all the observed forces. You could

  1. try to increase the range -frnn=… a bit, as long as grep svsl vol_0/fc.out does not output unphysical values of the force constants.
  2. try to include another unstable mode in the fit (-fu and then -gu=…) (can be combined with 1)
  3. restart from scratch using an even bigger supercell.
  4. look at the dos (need to include -fn to force it) and perhaps the instability is very small so that it can be ignored. (-fn will just ignore the unstable modes a re-scale the weight of the other modes).
3

Can we use the “-fn” option to force the calculation with unstable modes?

4

Yes, but it’s not a magic fix. If there are very few unstable modes and they are likley just numerical glitches, it gives reasonable answer. But, say, if you have 5% or 10% of modes that are unstable, that would not be a good fix.