quantum-espresso/examples/example15

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.