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README
Example for calculating the Raman tensor This example shows, how you can calculate the nonlinear susceptibilty and the Raman tensor with the help of the 2n+1 theorem. Therefore, different calculations are performed. First, a normal self-consistent calculation of the groundstate. Second, a response calculation for electric-field and atomic-displacement perturbations. All these calculations mentioned above are done in the standard way like in the calculation of phonon frequencies and the dielectric constant. Then a non-self-consistent calculation of the ground state is performed. Therin, the wavefunctions of a standard k-point mesh are computed, which is given in the usual way in the input of PW by K_POINTS. Additional, for each k-point, points k+b are calculated, where b is a small vector either in cartesian directions (lcart=.true.) or in direction of the next neighbour of the k point (lcart = .false.). The length of these vector is specified in the variable b_length. Finally, the perturbed wavefunctions are calculated non self consistently on the enlarged mesh (the variables lboth, lelfield, and latomic specify the way of calculating the derivatives with respect to the k vector by finite differences and therefore which wavefuntions are needed on the enlarged mesh), then the nonlinear susceptibility, and the Raman tensor.