Vibrational, Translational, Rotatioanl modes

Questions regarding the compilation of VASP on various platforms: hardware, compilers and libraries, etc.


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kambiz
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Vibrational, Translational, Rotatioanl modes

#1 Post by kambiz » Sun May 01, 2011 9:51 am

Hi dear all,

Frequency calculation of NH3 in the gas phase shows that there are 12 different modes. 6 for vibration, 3 (smaller than 20cm-1) for rotation/translation and 3 (imaginary) for rotation/translation.

On the surface there are 12 different frequencies:
3526, 3505, 3382, 1600, 1579, 1149, 676, 653, 395, 205, 135,
35i (imaginary)

My question is, should I consider that there are 11 vibrations or 1 rotation, or ... ?

I really appreciate your help

Cheers
Kambiz
Last edited by kambiz on Sun May 01, 2011 9:51 am, edited 1 time in total.

alex
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Vibrational, Translational, Rotatioanl modes

#2 Post by alex » Mon May 02, 2011 6:11 am

Hi,

there are _only_ 6 vibrational modes as you pointed out correctly. You need to setup the projection matrices (normally called A and B matrix if I'm not mistaken. Following the book of Wilson, Decius and Cross).

You night also try out phonopy (http://phonopy.sourceforge.net/), if the feature is implemented.

Or: there is a tricky little program derived from some CPMD stuff at my place. :-)

Cheers,

Alex
Last edited by alex on Mon May 02, 2011 6:11 am, edited 1 time in total.

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Vibrational, Translational, Rotatioanl modes

#3 Post by alex » Mon May 02, 2011 7:13 pm

Sorry, forgot to tell: Your low lying frequencies are far off anyway.
Check for small gradients and/or large enough simulation cell first.

Cheers,

Alex
Last edited by alex on Mon May 02, 2011 7:13 pm, edited 1 time in total.

admin
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Vibrational, Translational, Rotatioanl modes

#4 Post by admin » Wed May 11, 2011 5:03 pm

please check
1) that your geometry really corresponds to the equilibrium geometry (imaginary frequencies may indicate an exit channel from a transition state)
2) the displacements of the atoms (POTIM) are small enough to make sure you stay in the harmonic limit (POTIM= 0.015 or even smaller)
Last edited by admin on Wed May 11, 2011 5:03 pm, edited 1 time in total.

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