NLO Tutorial

PART1-DFPT

Five different steps:

1. alas_nonlin_ng6.in
- linear and non-linear responses to E field and atomic displacements
- generates 2 DDBs

2. alas_strain_ng6.in
- strain perturbation
- generates 1 DDB

3. mrgddb.in
- merge the 3 previous DDB into alas_DDB

4. anaddb.in
- analyse the previous DDB to get various responses

5. alas_fdiff.in
- compute d chi / d tau by finite differences
- deduce the Raman coefficient a and the Faust-Henry coefficient
- include some tools to compute the coefficients
