Vibrational, Translational, Rotatioanl modes

[kambiz]

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

[alex]

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

[alex]

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

[admin]

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)