How to find the vibrational frequencies using Pwscf? I have taken Experimental Raman spectrum. I have to assign the modes (A1g, B1u,B2g and A2g etc) for each peak in spectrum. Example given below.
Why do you not use Dalton, gamess, NWChem or Gaussian for calculating the vibrational frequencies??
It is no easy to do assign with your spectrum.
For easy way you needed:
- First, obtain a polarized spectra in all components scattering tensor.
- Second, you should do Group-Theoretical Analysis and compare result by component with you spectra. You can use for this, for example, Bilbao Crystallographic Server (http://www.cryst.ehu.es/ ) section "Raman and Hyper-Raman scattering". It's free.
You could do assign after calculations too, but you should do Group-Theoretical Analysis for it either.
What's your material? The nature of the material has some influence on which tools you use to model it. I agree with Alexander Krylov that a symmetry analysis is needed but if you have something related to a well-known system, you might find that is already published anyway.
Using PWSCF to calculate vibrational frequencies requires several steps, all of which need to be made with care to ensure good results. As Pavel Ignatiev says, you can download examples from the Quantum Espresso web site but I would add that you cannot jump straight to the calculation of phonon modes; you will need to ensure that you can obtain a well-converged total energy calculation for your structure, followed by an optimisation of the unit cell parameters and atomic coordinates, before you are ready to calculate phonon frequencies. So, you will need to work through several stages of examples. If you have a colleague who is familiar with the use of DFT codes, their advice will probably be very helpful. Failing that, there are online tutorial presentations on the Quantum Espresso web site that you should read.
Dear Kotagiri,
You can use a curve fitting program to obtain the main components of your experimental spectra. Then you can make assignment based on ab initio / DFT or other theoretical calculations (e.g. with Gaussian, as it was mentioned above). Or, you can find calculations in the literature. For curve fitting you may find free software from the Internet. Curve fitting of experimental spectra and assingment based on quantum chemical calculations of theretical spectra are two different things.
Dear Ildikó Ziegler,
I am doing curve fitting of experimental spectra by using origin software. Could you tell me any other software names?
Dear Daniel Wolverson ,
I am currently working on creating input file using PwScf. I have to get the optimization values then I will do SCF calculation for getting energy.
Thank you for giving valuable suggestions.
Hello Kotagiri, The vib. frequencies can be determined theoretically from the well known equation which uses the reduce mass of two atoms, and thend force const. of the bond. I think that your samples are some sort of oxides or alloys. you can search for the characteristic vib. bands for these samples in the appropriate literature. Best wishes
The non-polarized Raman spectra do not enough to assign the modes (A1g, B1u,B2g and A2g). You need in polarized spectra or in papers, where there is such assignment for our substances.
In particularly, B1u is IR-active only.
Pwscf (Plane-Wave Self-Consistent Field) is a wide class of ab initio software for simulations of molecule and solid state properties; there are both commercial and free examples. Normally they give both frequencies and irreducible representations (like A1g, B1u etc.) of simulated modes, and with some luck you can find correlations of experimental and simulated frequencies. Besides you can find such attribution using just symmetry of your sample and experimental geometry. In this latter case you will need information on your sample (is it gas, liquid, ceramics, crystal...), its symmetry (point group of a molecule or crystal), and obtain polarized Raman spectra.
B1u seems to be infrared active, indeed?
Use Gaussian to calculate de vibrational spectrum.
What is the structure of your compound?
Hi, to my opignon, you need first to read about the existing peaks of your material in the literature. Secondly, according to the selction Raman rules (geometrical configurations) given usually in the literature you could be able to affect the different peaks found in your measureements, and then use a lorentzian damped oscillator afected by a Bose-Einstein factor (the case of crystals) or a gaussian (for the case of amorphous materials such as liquids or gases) approach to fit the obtained spectrum. Unfortunatly, I am not, personnally, used to use this kind of materials. Good luck, yours R.H.
The excellent answers provided have probably answered your questions. I'd like to add though that in your calculations you may want to consider the crystal environment of your compound: the vibrations calculated for a single molecule by an ab initio approach may not account for extra bands introduced by possible site group and factor group splitting.
General selection rules give 3 set of Raman-active vibrations for orthorhombic
Ag, B1g,B2g and B3g
A, B1,B2 and B3
A1, A2,B1 and B2
Excelent answer Min Huang. I am going to study my materials theorically using this tutorial. Thank you.
There is a lot of free software on this subject in Internet. Try this one, for example:
http://www.cryst.ehu.es/
The quantum-espresso package, to which pwscf belongs, contains also a code called "PH" (for phonons). PH calculates vibrational frequencies using density functional perturbation theory. If you have a look at the long list of examples provided with the code, you will find the one that suites you. Note that Raman intensities can also be calculated using the 2n+1 theorem.
or, probably more straightforward than using the QE ph, you could use vasp with the isif=5 option, which generates the q=0 frequencies and eigenvectors. you can then symmetry-analyze the latter, and tell apart Raman and IR and silent. you can also extract IR intensities that way (you'll need effective charges, though ionic charges are ok for a rough assessment, and I can provide a fortran snippet). of course Raman intensities will require DFPT as implemented in QE or abinit.
That also depends on your budget or on that of your institution, since QE and abinit are distributed on GPL license, while for Gaussian and VASP you have to pay...
How I can insert parameters in vasp code to calculate vibrational properties.
Thanks
In The Name Of GOD
Hello
If you use the command #HF Freq test in Gaussian software, you will be able to calculate the Raman and Infrared spectrum. Please note that, you should put the command %Chk=file.name on the top of the Gaussian command. Then you should open the %chk=file.name through scratch directory of Gaussian using Gauss-View software. Then you can see the graph of Raman and Infrared an compare the vibrational modes with experimental data.
Best Regards
H.Simchi
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