I have a metallic structure. I have run geometry optimization first with 550eV energy cutoff, 0.02 k-mesh, and ultrasoft PP. Then when I tried to run a Phonon calculation with IR frequencies calculation it always failed. I then did a phonon calculation specifically for metal (clicking the checkbox in the CASTEP Calculation setup tab), and it was successful. But there were large -Ve frequencies for acoustic bands (~9.8 THz). So, I wanted to check the IR frequencies corresponding to the -Ve frequencies. But if I select both metal and IR calculation (as well as polarisability), an error message pops up like the below - 
Now, I performed an IR frequency calculation with the resultant phonon structure, but there was no -Ve frequency.

Now, how can I perform Phonon as well as IR frequencies for my metallic structure?