deletions | additions       diff --git a/Sternheimer2.tex b/Sternheimer2.tex  index 4bf5d02..dd1c77e 100644  --- a/Sternheimer2.tex  +++ b/Sternheimer2.tex 
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\begin{equation}  \tilde{Z}^{*}_{i j \alpha} = Z^{*}_{i j \alpha} + \frac1N\left( Q_{\rm tot} \delta_{ij} - \sum_{\alpha} Z^{*}_{i j \alpha} \right)\ .  \end{equation}  The discrepancy arises from the same causes as the non-zero translational and rotational modes. By mixing the response to ionic displacements and electric perturbations it is possible to calculate vibrational Raman response coefficients~\cite{PhysRevB.66.100301}. This feature however, it is still not implemented in Octopus.  The Sternheimer equation can be used in conjunction with $\vec{k} \cdot \vec{p}$ perturbation theory~\cite{Cardona_1966} to obtain  band velocities and effective masses, as well as to apply electric fields via the quantum theory of polarization. In this case the perturbation is a displacement in the \(k\)-point. Using the effective Hamiltonian for the \(k\)-point