:) I'm bit surprised that so far nobody recommended reading a book or at least a chapter on group theory. Of course you can use any of the recommended packages to quickly assign to which irreducible representation a given normal mode corresponds and also to visualize these vibrations (which I agree is quite helpful). However, in order to understand what those labels actually mean and to be able to assign them without any software (which is hardly a rocket science and in this case should be a piece of cake since the point group is only CS) you need to consult a textbook. For a brief introduction I would recommend the chapter on molecular symmetry in P.W. Atkins Physical Chemistry, which focuses on MO's, and any introductory Spectroscopy (for instance Hollas, Modern Spectroscopy). Finally, for a more elaborate lecture see Cotton's Group Theory, which is a classic. Then perhaps you realize that you do not need any quantum chemistry package to perform this task. I can recommend also the J.P. Goss page on point group symmetry [http://www.staff.ncl.ac.uk/j.p.goss/symmetry/]

These symbols come from Character tables.

You can study the book: Chemical Application of group theory by F. Albert Cotton.

How can it be defined using Gaussian calculated file?

I am using Dalton electronic structure program...

In which, we generally optimize the geometry...Where the program itself computed the symmetry and point group.

Using that optimized geometry, we use for further applications..

OK, I'll try that........thank u so much Dr. Alamgir Khan

You are well come..

Molecular Symmetry and Group theory - A programmed introduction to Chemical Application by Alan Vincent

It is a very simple short study guide or book to understand the Group theory and its applications to chemical bonds and molecular vibrations.

it is easy to define them on the basis of symmetry. you can check the book of cotton or Vincent in which it is derived how to get these terms

for small molecules....

You can perform electronic structure calculation with gaussian program. You can optimize and do frequency

analysis to get those normal modes with there assigned symmetry.

ok Prashant Gupta ji............I think the information given by you is valuable.....but from .OUT file of gaussian03 programme (opt+freq), I am not able to see the symmetry.

the little portion of the file is as-

109 110 111

A A A

Frequencies -- 3067.5436 3103.9875 3108.3528

Red. masses -- 1.0576 1.0953 1.1081

Frc consts -- 5.8634 6.2175 6.3079

IR Inten -- 18.3900 18.4379 7.3874

Atom AN X Y Z X Y Z X Y Z

1 7 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

2 6 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

.................................................................................................................

....................................................................................................................

You can find the symmetry details in the section stoichometry of gaussian out file.

Some times, the program do not give the expected group point e.g for CH4 molecule, the program do not give D4h point group, it gives a lower symmetry group like C2v, Cs etc.

But you can adjust the states using correlations tables.

Dear sir, I do not want define the symmetry of molecule, here I am talking about symmetry in stretching mode (vibrational mode). Some papers of spectrochimica acta write them in the IR and RAMAN table in the side of calculated frquencies. I have attached a file here. In this file at page number 112 the table given with symmetry of vibrational mode. please see it.

The information about the point group (symmetry) of the molecule is shown in stoichiometry part of the output file

apart from this there are information for symmetry of each molecular orbital......like A1g, B1g....and these are defined depending upon whether molecule has center of symmetry or not, mirror symmetry and other symmetry as well....

these flags are assigned on the basis of present symmetry for these modes, because during vibration the center of mass of molecules is preserved. u and g subscript correspond to ungerade and gerade which means that during vibration molecule preserve center of symmetry or not, similarly 1 and 2 correspond to plane of symmetry.......there is an excellent book by indian professor on group theory and symmetry by K Veera Reddy

For finding the type of vibrations, we generlly use the Opt+Hessian out file, and check the type of vibrations in molecule designing or constructing program i.e MOLDEN or Molplt or Gauss view etc . For Dalton program we take the hessian computed .dat file. From there, you can see the type of vibrations...

Type of vibrations = normal modes

If u use the checkpoint file or output file in gauss view then u will find the symmetry associated with a particular

mode...

Mr. Prashant Gupta, which section gauss view shows the symmetry of modes.........I am not able to find that

In molden, we use the normal mode option, Which read the .out file of gamess and dalton.

In case of molplot, we use the .dat file of gamess.

Hi

you can impose the symmetry on your compound before that you start your work with ChemCraft software.

Gaussian detects automatically the molecular symmetry. The input geometry must satisfy the expected symmetry up to a certain threshold (hardcoded value?). If using GaussView, look for the Symmetrize tool (an icon with a water molecule and a symmetry plane). Also try helping Gausssian/GaussView fo find the desired symmetry group: adjust/equalize bond lengths and so on. Computational chemistry reside actually behind graphical interfaces. Try searching "Frequencies --" in the log file of your frequency job and look one line above. Irreducible representation of the corresponding vibration is printed below the mode number and above the vibration wavenumber. In C1 spacegroup (no symmetry) an A (A1) will be printed for each frequency.

I suggest to use the TURBOMOLE program. Its the fastest code to compute the normal modes and IR and Raman selection rules and intensities at the DFT level with various functional. Its input generator will automatically detect the molecular point group and the program for computing the normal modes automatically prints out the symmetry labels together with the frequencies and intensities for each normal mode.

:) I'm bit surprised that so far nobody recommended reading a book or at least a chapter on group theory. Of course you can use any of the recommended packages to quickly assign to which irreducible representation a given normal mode corresponds and also to visualize these vibrations (which I agree is quite helpful). However, in order to understand what those labels actually mean and to be able to assign them without any software (which is hardly a rocket science and in this case should be a piece of cake since the point group is only CS) you need to consult a textbook. For a brief introduction I would recommend the chapter on molecular symmetry in P.W. Atkins Physical Chemistry, which focuses on MO's, and any introductory Spectroscopy (for instance Hollas, Modern Spectroscopy). Finally, for a more elaborate lecture see Cotton's Group Theory, which is a classic. Then perhaps you realize that you do not need any quantum chemistry package to perform this task. I can recommend also the J.P. Goss page on point group symmetry [http://www.staff.ncl.ac.uk/j.p.goss/symmetry/]

Dear Vikas,

First define symmetry (C1, Cs, C2v etc) of the molecule using GaussView (go to Edit-Point group and tick on enable point group symmetry and always track point group symmetry) or any graphical interface. After that perform calculations using keyword OPT+FREQ. You will surely see the symmetry elements correspond to each vibrational modes in gaussian out put file. By default C1 symmetry is used for molecule in Gaussian.

In the reference which you have attached, the calculations are done by assigning point group symmetry (C1-only E, Cs-planar molecule) on the molecule. Cs point group contain a' (in plane vibrations) and a''(out of plane vibrations) symmetry elements. C1 point group is applied on that molecule which do not have any symmetry except E.

good luck

Thank you Dr. Robert Góra , I am really impressed by your answer, I think this is the best way. Thank you so much. I'll try to find out the solution of my problem.

Atkins explained all about molecular symmetry with good ranging from basics to advanced concepts