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Hello,

I have two questions:

1. When I use IBRION=5 in the INCAR file to calculate the frequency of my system, are the vibrations of my system given by its normal modes at the end of the OUTCAR file?

2. For a system, e.g., a hydrogen molecule adsorbed on a Ni(111) surface, I only want to know the vibrational frequency of the hydrogen molecule, and those of the Ni surface are not needed. I wonder if I can fix all Ni atoms using the "selective dynamics F" in POSCAR, and if this treatment may induce any errors of the calculated frequency. (I think normal modes are independent; they don't interact. right?)

Thank you very much for your kind attention.

yes

From the guide, I know that IBRION = 5 can only calculates gamma-point frequency. So, I wonder if I must set 1X1X1 mesh in the KPOINTS file. And if the answer is No, do other k-meshes (e.g., 2X2X2) lead to different frequency results (more precise)? Thanks.

What you set in KPOINTS is the k-mesh for electronic calculations, while by setting IBRION = 5, you can calculate the vibrational frequencies at the q-point of Gamma. It is different from each other.