Usually you can apply the symmetry when you make the molecule by Gaussveiew (from mono choose edit then point group and choose appropriate tolerance) and run by Gaussian 09. If this did not work, you must manually select the appropriate variables. For example, in H2O you should write the same variable for both OH distance:

O                                                                                                                                                      H, 1, R1                                                                                                                                       H,1,  R1,2 A2

R1 0.96                                                                                                                                          A1 105.0 

Hello Turbasu!

Kindly follow the periodic boundary condition (PBC) calculation method to compute the molecule in symmetry

http://www.gaussian.com/g_tech/g_ur/k_pbc.htm

http://scuseria.rice.edu/gau/PBC-Guide.pdf

http://www.scuseria.rice.edu/gau/gdv_pbc/pbc_guide.html

I hope this will HELP

Thanks

Dear Turbasu,

On my knowledge, Gaussian is not prone to change point group during optimization. Could you attach your output file?

Maybe, your starting geometry was not a Td. Gaussian is too exacting to a Td group specification. In particular, one should specify the bond angle to be 109.4712 (not 109.5!). Please find corresponding log file for methane optimization attached.

Dear Vladimir,

My molecule is a bit big. So, Z matrix specification is not possible. I've attached  my output file. Before start of the optimization cycle symmetry of the molecule is Td which changes to C1 on-wards .There is some warning about "symmetry breaking in density matrix" and "Density has only Abelian symmetry"( line number 1000) which may be the reason but I'm not sure.

Although it might take some time, it definitely is possible to specify a z-matrix for this molecule, especially as symmetrical as it is. That is one solution. I am working from your output file to see if G09 has a combination of keywords that will maintain the high symmetry during optimization. I'll let you know what I find.

I'm not sure why my first answer got voted down :-( I was just restating what someone had correctly said before - that the best solution was to use a z-matrix with symmetry-equivalent coordinates labeled the same!

The second part of my first reply was that I would re-run the molecule with some additional keywords to see if G09 would maintain the Td symmetry. I have done that, but the molecule still breaks symmetry after the first point. So my suggestions are, 1) again, take the time, use a good geometry, and create a z-matrix in Td symmetry and 2) ask for help on the Gaussian web site.

Good luck!

These are from the Internal Option Reference and they might help you. All you have to do is add the IOp(s) to the command line like so: IOp(2/15=4,2/16=3)

IOp(2/15)

SYMMETRY CONTROL.

-1 Turns on symmetry; same as 0 for molecules but turns on assignment of space group ops. for PBC.

0 Leave symmetry in whatever state it is presently in.

1 Unconditionally turn symmetry off. Note that symm is still called, and will determine the framework group.

However, the molecule is not oriented.

2 Bring the molecule to a symmetry orientation, but then disable further use of symmetry.

3 Don't even call symm.

4 Call Symm once with loose cutoffs, symmetrize the resulting coordinates, then confirm symmetry with tight cutoffs.

5 Recover the previous symmetry operations from the rwf, and confirm that the new structure has the same symmetry.

6 Same as 5, but get symmetry info from the chk.

00 Default (10)

10 Do re-orientation for PBC.

20 Suppress re-orientation for PBC.

100 Turn on symmetry operations for PBC.

IOp(2/16)

action taken if the point group changes during an optimization.

0 Abort the job.

1 Keep going.

2 Keep going and leave symmetry on, using the old symmetry.

3 Keep going and leave symmetry on, using the new symmetry.

I am in difficulties to fixed symmetry of the compound 1,5 dihydroxy 9,10 anthraquinone , a highly symmetric C2h system. Optimization for the transition state of this compound in B3LYP/6-31g(d) method leads a bent molecular system with loss of aromaticity. I draw the initial structure through Gaussview. The framework group has been found to  C1[X(C14H8O4)].

How can I solve this problem relating to fixed symmetry.

You must also remember to use z-matrix instead of Cartesian coordinates, and also apply opt=z-matrix

Sir, As per your advise I tried with simplest system ,Benzene, but it also failed to find the point group of benzene i.e. D6h. I am sending you the input. How can I solve this?

Actually, the 1,5 dihydroxy anthraquinone system to which I want to calculate the transition state with 2 strong Hydrogen bond, became bent during transition state optimization and show as if the aromaticity of  anthraquinone being lost.  But literature on 1,5 dihydroxy anthraquinone  shows a perfect planar structure with C2h symmetry.

The IOp should work, but if didn't there is another way. If you are using gauss view to draw your molecule you can use the clean tool to adjust the geometry. Use the symmetrize tool find the highest point group, then use the point group tool to constrain it in the highest one. After you generate the input add the key word geom=connectivity and make sure that the connectivity matrix was add after the z-matrix.

I am trying to do a geometry optimization. The input contains only 6 atoms, and the output should be in C3V symmetry structure. I have given the input in the same symmetry, but the symmetry changes. I have tried using iop commands as suggested by Cory Camasta . But no help.

Any ideas?

Z-matrix, but keep in mind that after applying to system any constraints you cannot be sure that optimised structure is true minimum.

If all other options fail, maybe the method/basis is just not suitable...

... Or the molecule is not really C3v. I assume you've already found some experimental data to verify that this is the result you should be getting...?

@prasanta bandyopadhyay

It is obvious from the description of gaussian's layer 2/16, that if you want to maintain the strictly definite type of symmetry (e.g. Td in this case), you need to set IOp(2/15=4,2/16=2) instead of IOp(2/15=4,2/16=3) because:

Op(2/16)

2 Keep going and leave symmetry on, using the old symmetry.

3 Keep going and leave symmetry on, using the new symmetry.

Thus, even if Td will be broken to lower type of symmetry, the calculation will turn back to Td, while parameter "2" will result in the following of new type of symmetry.

Cory Camasta Not exactly, I do not have experimental data. But I have a feeling that the desired output structure (as well as input) should be C3v. I have C, N, H, I atoms and tried using aug-cc-pvtz (PP) and def2-tzvpp basis sets with RI-MP2 and m062x. I am trying to reproduce and modify a work by Clark as linked below. However, the structure becomes heavily distorted sometime.Article The Interaction of Radicals with σ-Holes

When you have Jahn Teller effects symmetry breaking occurs automatically. One cannot avoid it.

Laszlo Nemes

Budapest