Small Imaginary Frequency

[Sophie]

Hi All,

I have been trying to optimize a dimer system using BP86-d3/def2-tzvp. After several trials,  I am still getting small imaginary frequency, around -7 or -8 cm-1.

Below is the input file:

start molecule

title "Optimization dimer"
charge 0

memory 9000 mb

geometry autosym 0.2
S       -3.1956081700     -0.3873016900      0.7180951400
F       -4.2690340100     -2.3082717300      2.2488347600
F       -3.8671155900     -2.9238135100      0.1826436700
F       -5.5773710500     -1.6248059100      0.6277161000
O       -3.2890678400     -0.1079870700     -0.7254315000
O       -3.7656946100      0.6329671400      1.5970830200
O       -1.8654108100     -0.8994694600      1.1280073100
C       -4.2986103200     -1.8932098600      0.9601464200
S        3.1956081700      0.3873016900     -0.7180951400
F        4.2690340100      2.3082717300     -2.2488347600
F        3.8671155900      2.9238135100     -0.1826436700
F        5.5773710500      1.6248059100     -0.6277161000
O        3.2890678400      0.1079870700      0.7254315000
O        3.7656946100     -0.6329671400     -1.5970830200
O        1.8654108100      0.8994694600     -1.1280073100
C        4.2986103200      1.8932098600     -0.9601464200
N        0.4888365200      0.8943232800      2.8158299400
N       -0.7593548100      1.8994659900      1.3526351600

C        0.3559369400      1.1744331700      1.5097162100
H        1.0364605700      0.8950410300      0.7060277700
C       -0.5839423500      1.4434710400      3.5029793800
H       -0.7017782200      1.3164193400      4.5718468500
C       -1.3681131600      2.0784033700      2.5794742700
H       -2.3186171700      2.5851079800      2.6755059300
C        1.5710697000      0.0811564400      3.3869588000
H        1.7349342500      0.4508573200      4.4110512700
H        2.4727957100      0.2870226900      2.7950527800
C        1.2803514100     -1.4072496800      3.3932611400
C        2.3659174500     -2.2938876100      3.3362083800
H        3.3773169200     -1.8998322600      3.2131725200
C        2.1534021600     -3.6728416500      3.4056544200
H        3.0030184500     -4.3551766400      3.3449352000
C        0.8552991200     -4.1784319300      3.5206407100
H        0.6861197300     -5.2559974500      3.5375721900
C       -0.2289521400     -3.2987680200      3.5450188000
H       -1.2482092300     -3.6850245400      3.5840241000
C       -0.0209218800     -1.9177016500      3.4836275500
H       -0.8835264600     -1.2510177400      3.4504464600
C       -1.2182241900      2.4386619700      0.0577634700
H       -2.3123252200      2.3635375500      0.0420253000
H       -0.8261193800      1.7698525800     -0.7195025700
C       -0.7254533700      3.8494696900     -0.1581952400
C        0.6372859800      4.0741243200     -0.3981727200
H        1.3258560800      3.2271230900     -0.4444303400
C        1.1045704400      5.3694193300     -0.6232075900
H        2.1662465500      5.5328917800     -0.8163509400
C        0.2117313900      6.4477631300     -0.6200462900
H        0.5762449700      7.4602715600     -0.8042538900
C       -1.1483738800      6.2267365800     -0.3829018400
H       -1.8482038100      7.0645577300     -0.3822587600

C       -1.6143114200      4.9291623900     -0.1477553700
H       -2.6785303800      4.7508823900      0.0246553200
N       -0.4888365200     -0.8943232800     -2.8158299400
N        0.7593548100     -1.8994659900     -1.3526351600
C       -0.3559369400     -1.1744331700     -1.5097162100
H       -1.0364605700     -0.8950410300     -0.7060277700
C        0.5839423500     -1.4434710400     -3.5029793800
H        0.7017782200     -1.3164193400     -4.5718468500
C        1.3681131600     -2.0784033700     -2.5794742700
H        2.3186171700     -2.5851079800     -2.6755059300
C       -1.5710697000     -0.0811564400     -3.3869588000
H       -1.7349342500     -0.4508573200     -4.4110512700
H       -2.4727957100     -0.2870226900     -2.7950527800
C       -1.2803514100      1.4072496800     -3.3932611400
C       -2.3659174500      2.2938876100     -3.3362083800
H       -3.3773169200      1.8998322600     -3.2131725200
C       -2.1534021600      3.6728416500     -3.4056544200
H       -3.0030184500      4.3551766400     -3.3449352000
C       -0.8552991200      4.1784319300     -3.5206407100
H       -0.6861197300      5.2559974500     -3.5375721900
C        0.2289521400      3.2987680200     -3.5450188000
H        1.2482092300      3.6850245400     -3.5840241000
C        0.0209218800      1.9177016500     -3.4836275500
H        0.8835264600      1.2510177400     -3.4504464600
C        1.2182241900     -2.4386619700     -0.0577634700
H        2.3123252200     -2.3635375500     -0.0420253000
H        0.8261193800     -1.7698525800      0.7195025700
C        0.7254533700     -3.8494696900      0.1581952400
C       -0.6372859800     -4.0741243200      0.3981727200
H       -1.3258560800     -3.2271230900      0.4444303400
C       -1.1045704400     -5.3694193300      0.6232075900
H       -2.1662465500     -5.5328917800      0.8163509400

C       -0.2117313900     -6.4477631300      0.6200462900
H       -0.5762449700     -7.4602715600      0.8042538900
C        1.1483738800     -6.2267365800      0.3829018400
H        1.8482038100     -7.0645577300      0.3822587600
C        1.6143114200     -4.9291623900      0.1477553700
H        2.6785303800     -4.7508823900     -0.0246553200

end

basis spherical
  * library Def2-TZVP
end
basis "cd basis" spherical
  * library "Weigend Coulomb Fitting"
end

relativistic
  zora
end

dft
  xc becke88 perdew86
  disp vdw 3
  direct
  grid xfine nodisk
  convergence energy 1d-10 diis 80 ncyds 80 damp 0
  print low
  tolerances tight
  iterations 1000
end

driver
  clear
  tight
  maxiter 500
end

task dft optimize
task dft freq numerical

The output file shows 1 imaginary frequency, as follow: 

         -------------------------------------------------
          NORMAL MODE EIGENVECTORS IN CARTESIAN COORDINATES
          -------------------------------------------------
             (Projected Frequencies expressed in cm-1)

                    1           2           3           4           5           6

 P.Frequency       -7.66       -0.00       -0.00        0.00        0.00        0.00

In the other input file, instead of using   convergence energy 1d-10 diis 80 ncyds 80 damp 0, I used convergence energy 1d-10 damp 45 ncydp 0 dampon 1d99 dampoff 1d-4 while keeping the rest the same, but still I obtained one small imaginary frequency at -8.27 cm-1 in the output file.
I ran the calculation using NWChem 6.8.

Any help or tips on how to get rid of this imaginary frequency would be greatly appreciated.

Thanks,

Sophia S

 

[Edoardo Aprà]

Sophia

I have run a few tests and this numerical instability seems to be caused by selecting the perdew86 correlation functional.

Can you try to run using

xc xpbe96 cpbe96

?

[Daniel Mejia Rodriguez]

Edo,

I think that the DFT-D3 gradients (both vdw 3 and vdw 4) are slightly wrong. One thing was the inclusion of three-body terms in the gradient, but I still see some differences between a finite-difference approach and the analytic DFT-D3 gradients implemented. Maybe this is the reason for the small imaginary frequencies?

Daniel

[Edoardo Aprà]

Yes, I thought that could have been the case, too.

In order to factor vdW corrections out, I have run this molecule without any corrections with (xc)-pbe96 and xbpe96-perdew86

The (xc)-pbe96 case gave zero imaginary frequencies, while the xbpe96-perdew86 gave one

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[phia....@gmail.com]

Hi Edo,

Thanks for input. I shall try to run as suggested.

 

Sophia

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