[gpaw-users] normal mode calculation via ase

[Jay Wai]

Dear users,

I am trying to calculate the free energy of a h2o gas molecule, but I am
stuck in the step of normal mode calculation because of imaginary
frequencies.

My code is as follows:

calch2o = GPAW(mode=PW(800), xc='PBE', kpts={'size': [3,3,1]},
symmetry='off', convergence={'energy':0.0001, 'density':1e-6}, basis='dzp',
txt='h2o.txt')
h2o = molecule('H2O')
h2o.set_cell([15,15,15])
h2o.set_pbc([1,1,1])
h2o.center()
h2o.set_calculator(calch2o)
opth2o = BFGS(h2o, trajectory='h2o.traj')
opth2o.run(fmax=0.02)
vibh2o = Vibrations(h2o)
vibh2o.run()

I have run a similar code for other molecules, including co, h2, co2 and
found that imagnary frequencies disappear for those molecules as I
increased the cut-off energy and the number of kpts.
That's why I set the cut-off energy to be so high (800) and included
several k-points for a single molecule calculation.

The result of normal mode analysis is as follows:
---------------------
  #    meV     cm^-1
---------------------
  0    7.8i     63.3i
  1    7.1i     57.0i
  2    1.8      14.2
  3    3.1      25.3
  4    5.2      41.6
  5    7.8      63.0
  6  197.4    1592.5
  7  460.8    3716.8
  8  473.4    3817.9
---------------------

My quesiton is how I can change my code not to get imaginary frequencies.
I've tried the followings, but none of them worked.
a) set nfree to be 4 when calling Vibrations()
b) set energy convergence criteria to be 1E-6
c) try other cut-off energies: 400, 500, 600
d) set fmax to be smaller values in geometry optimization

Can anybody help me on this?

Or, can I just use the last three frequencies and call ase.IdealgasThermo
function because we know that h2o only have three normal modes of vibration?

Thank you,

Jay
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