[QE-users] how to select the perturbed atoms in phonon single q calculation ?

[Omer Mutasim]

Dear all
 I need to calculate the the virbrational frequencies of adsorbate molecule on surface using phonon single q calculation  , in order to estimate the partition function (for entropy ,reaction rate constants). so my questions go like:
 I have a large supercell (110 atoms) which means a high degrees of freedom (330 DOF) ,  so i want to decrease this DOF , by perturbing only adsorbate molecule and the the two uppermost layers
how to select the perturbed atoms in quantum espresso ?I have heard that it can be done by finite difference method, which wasn't employed in QE. However, i have seen a post where Dr. Paolo Giannozzi said: " it can be performed by two finite-difference calculations with opposite displacements "So , can you please tell me, what are the steps involved in doing this finite-difference method mentioned by Dr. Paolo ? or any other procedure that can be do the same ? 
 Thanks in advance



Omer Elmutasim Research AssistantChemical Engineering DepartmentKhalifa university- UAE
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