IFC_real sapce

[Dipankar Saha]

How can I calculate the interatomic force constants in real space ??

Best_
Dipankar

[Jess Wellendorff]

This is exactly what the dynamical matrix (DM) is:
http://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.dynamicalmatrixparameters.html. Only question is which components of the DM you need. Remember that the eigenmodes of the DM are characteristic vibrational modes of the crystal: http://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.vibrationalmode.html.

[Dipankar Saha]

I need to find ...the short range chemical forces.... / To be more application specific... I want to have a number which will give me the IFC  value between the two adjacent layers for any multi layered structure.

Thanks and Regards_
Dipankar

[Jess Wellendorff]

Hmm, that sort-of implies that you want to consider the atom within each layer fixed in their layer, and then look at the force between such "rigid bodies" of atoms. That, at least, would be the approach in a molecular dynamics simulation.

[Dipankar Saha]

Does it need MD simulation....??!! / Would you please check the following link once......
http://www.quantum-espresso.org/wp-content/uploads/Doc/ph_user_guide/node8.html

[Jess Wellendorff]

No, I am not saying that you need to do a molecular dynamics simulation. What I am saying is that you need to figure out exactly what sort of force constant you are looking for. ATK and Quantum ESPRESSO can both calculate the full dynamical matrix, which for each atom has matrix elements with all other atoms and for several directions. Is one of these matrix elements the one you are looking for, or do you need some post-processing to derive an effective force constant between layers in a multilayered structure? That is your scientific task.

[Dipankar Saha]

I guess both.....however, the post processing to derive a relevant constant value  can still be done.... but, manually finding the required matrix element which exactly refers to the the  inter atomic force between two similar or dissimilar  atoms is really tedious (error-prone too) job !!! Is it not...??

Thanks a lot for your help Jess..!!! 

Regards_
Dipankar

[Jess Wellendorff]

Yes, might be tedious. Also, I am not convinced that a single matrix element is what you need. I don't think you can find an element of the dynamical matrix that can really be interpreted as the force constant between individual layers in a multilayer structure. I think you somehow have to look at the energy and forces in a system where you displace the layers from each other in a rigid manner.