 Phonon branch (source code)

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= Phonon branch
{title2=$\omega_s(\mathbf k)$}

A continuous family of <normal modes> labelled by the <wavevector> and a branch index $s$. For $r$ <chemical atoms> per three-dimensional <primitive unit cell>, the <dynamical matrix> has $3r$ <eigenvalues>, giving $3r$ branches: three <acoustic phonon branches> and $3r-3$ <optical phonon branches> for a mechanically stable ordinary <crystal structure>.

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