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Hello

In the OSZICAR output documentation page at wiki/index.php/OSZICAR there is a line about d eps column, which says "... and d eps the change in the bandstructure energy".

Would you, please, give a detail on what does bandstructure energy mean compared to, lets say, free energy, and how it is calculated?

And one more question, short one. Where can I find the force matrix after ionic steps per atom, or per kpoint?

Hello,

With bandstructure energy vasp means the sum over all occupied Kohn-Sham eigenvalues.
Say f_nk is the occupancy of band n at k-point k and e_nk the corresponding energy eigenvalue of the Kohn-Sham Hamiltonian, as explained
on page 15 of these lecture notes.
The bandstructure energy is then sum_nk f_nk e_nk and d eps is the difference of this value between the current and last iteration.

Look for "Hessian matrix" in the OUTCAR, which is the second derivative of the total energy with respect to atomic displacements).
Maybe following wiki entries are of help for you:

wiki/index.php/IBRION#IBRION.3D5_and_6: ... erences.29.
wiki/index.php/Phonons_from_finite_differences