# Fix Phonon

Dear Lammps users,

I am trying to understand the output of the fix phonon and tried to develop the dispersion relation of the Cu based on the file provided on Github. Regarding the Lammps documentation and the references, one should be able to find the frequencies by calculation the eigenvalues of the dynamical matrices, which are the result of the fixphonon. Unfortunately, the frequencies are not matching or even in close proximity of the values provided by the literature and the example on Github. My post-processing python code follows the following algorithm;

1. Creating complex numbers from the real and imaginary parts of the D matrix for each q vector
2. Creating a 3N*3N matrix from the numbers, by filling the rows till the matrix is fully filled
3. changing the unit from ev/(A^2grams/mol) (As the simulation was with unit metal) to J/(m^2Kg) by this factor (1.60218e-19/(1e-20*(512*1e-3)/(6.022e23)))
• 512 is the number of atoms in the simulation1. Calculating the Eigenvalues and Eigenvectors
1. finding the values for 100, 110, and 111 direction among the q values
• 111 direction IF qx, qy, and qz are equal
• 110 direction IF qx=qy and qz=0
• 100 direction IF qy=qz=01. Checking the maximum frequencies in the system