I have a molecule which gets optimised in D3h symmetry. I have also done the frequency calculation of that molecule. However, the symmetry of the frequency modes do not get printed and shows a ? in that place. How can I know the symmetry of these frequency modes without lowering the symmetry of the molecule?
The symmetry may have been broken (slightly) during optimization and orbital/vib mode symmetries are no longer pure. You probably have ? for molecular orbital symmetries as well(?)
To get rid of some of the ?, I would re-optimize with tight or even very tight optimization criteria. Also, tighten the SCF convergence criterion.
The ? often come in the degenerate modes and (obviously) come in pairs then. You can visualize the modes and determine whether they are 'g' or 'u'. Similarly,if you have to go to the D3 point group to get rid of the ?, 'g'or 'u' is most easily determined from GView
Yes, the symmetry of the molecule was being broken (D3h -> C1 and then it gets optimised in D3h). The molecular orbitals also contained ? in many cases. I was using the condition symm=veryloose in the input.
If the symmetry IS broken, then the optimization takes place in the lower symmetry point group (in your case apparently C1). The optimized structure is very close to having D3h symmetry, so the program tries to assign symmetry according to the D3h point group----and you get the ? etc.
I think you want to tighten all sorts of criteria (integral accuracy, scf convergence, opt=tight or verytight)---none of that 'loose' stuff.
This time I had used opt=Vtight along with freq in my input file. Also the symm=veryloose condition was removed. The result was the molecule got optimised in C1 symmetry instead of D3h (as happened earlier). All the frequency modes are of A symmetry, replacing the previously ?. The frequencies came at exactly the same wavenumbers. Another interesting point is that the energy difference between the D3h optimised molecule (previous one) and the C1 optimised molecule (new one) is of 5E-5 eV.
I have also tried figuring out the symmetry of the ? obtained previously. They came out to be E' : they really were degenerate as you had earlier said.
Many thanks Professor Karsten_Krogh-Jespersen for helping me out with this problem. Please do send your opinion based on the information I have mentioned aforesaid.
Dear Maitrayee,
in numerical calculations symmetry is accuracy dependent.
You can maintain D3H symmetry if you write the input file as a z-matrix, specifying exactly which bonds should be identical by symmetry, which angles etc.... and then use opt=z-mat. However, that can become quite tedious with larger molecules----but that's the way we did it in the good old days (1980's or so).
When Gaussian chooses the internal redundant coordinates (or is it redundant internal?) it wants to use, I don't think it is paying particular attention to the overall symmetry of the molecule. It specifies more internal coordinates than actually needed, and I think that somehow leads to small inaccuracies and inequivalencies, so that the symmetry is ultimately broken.
- Badges
- Science topic
- Similar topics
- Chemistry
- Theoretical Chemistry