Molcas Forum

chipmunk

Member

Registered: 2015-11-12

Posts: 23

[SOLVED] Just obtain 2 normal modes of vibrations instead of 3 in C2v?

Dear molcas developers and users,

I, recently, have been running some optimizations at CASPT2 levels for a few C2v symmetry molecules containing 3 atoms like CO2. There have been also vibrational calculations after the optimization calculations to confirm true minima. However, in stead of obtaining 3 vibrational normal modes as other programs do, &Mckinley just gave me two normal modes as the following example.

Harmonic frequencies in cm-1

 IR Intensities in km/mol

                        1         2
 
     Frequency:       470.35    606.12                                                                                  
 
     Intensity:    0.549E-03 0.872E+02
 
               z    0.09632  -0.05529
               y   -0.06656  -0.11594
               z   -0.07738   0.04443
 
 
WARNING: No thermochemistry analysis conducted for numerical frequencies unless no symmetry is used!

Is there something wrong in my calculations? Do I need to change something in the inputs to obtain three modes?

My Mckinley key

&Mckinley
Perturbation
Hessian

Thanks lots,

Nahn

Last edited by chipmunk (2016-08-02 11:06:27)

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,085

Re: [SOLVED] Just obtain 2 normal modes of vibrations instead of 3 in C2v?

I'm afraid that happens when you have symmetry and numerical frequencies, you only get the symmetry-conserving vibrational modes. If you think about it, it's only logical. In order to compute the frequencies numerically, several energy/gradient calculations have to be done at perturbed geometries. With perturbed geometries that maintain the symmetry, only symmetry-conserving frequencies can be obtained. To obtain all vibrations you'd need to compute energies/gradients at perturbed geometries with broken symmetry, but then you cannot do a calculation with symmetry at a geometry with broken symmetry.

So, be satisfied that your geometry is a minimum within that symmetry, or compute the frequencies without symmetry.

chipmunk

Member

Registered: 2015-11-12

Posts: 23

Re: [SOLVED] Just obtain 2 normal modes of vibrations instead of 3 in C2v?

Thanks a lot Ignacio. Now I understand the reason.

Have a nice day.

Nahn