I want to do DFT simulations of the full crystal structure as well as a single molecule of a chemical (organic). I have completed the full crystal simulations and want to check the changes in the frequencies if I do the DFT simulation with just a single molecule.
For the crystal structure, I knew the space group (I got it from the cif). But how do I find out the point group of a single molecule of the same chemical?
If you know the Z number of molecules per unit cell and this requires that your molecule is on a special position, you can get some information on the symmetry of the molecule, However you have a little chance of getting full symmetry of the molecule in this way.
For example, if the space group is P2/m and you have only one molecule per unit celle it must sit at the intersection of 2 and m, tus the molecule ha symmetry 2/m. If instead Z = 2, the molecule can have symmetry either 2 or m. Finally if Z = 4 the molecule is in general position and there are non constrains on its symmetry.
Dear Shaumik, I 've just read the answers to your question. I completely agree with the answers. Unfortunately, outside rare exceptions, the knowledge of the space group is insufficient ; you must know the structure : Wyckoff positions of atoms in the cell. Knowing only the symmetry elements is clearly insufficient. The example given by Giovanni is characteristic : every site in the cell has a determined point symmetry; thererfore, you must know what atom is where...
Dear Shaumik, if you have the CIF (crystallographic information file) - take coordinatesof the atoms of your molecule , identify Wykoff positions and their symmetry. This should help to find the point group of the molecules, if all the atoms sit in general position than you symmetry is just 1 irrespective of the space group of the crystal (this is what Giovanni has mentioned above ) . Also it may help to plot the structure in Mecury software which allows to see all the symmetry elements, then omit the elements multiplying molecules and leave only those that relate atoms within the single molecules - those symmetry elements form your point group. Software is free.
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