It is possible to study the excitations such as magnon or phonon. The problem is relatively simple in the case of a spin spiral but can become quite complex in the case of a non-collinear order parameter which is not commensurate with the unit cell.

Fourier transform of H in the chemical unit cell (FT)

It is possible to calculate the FT of the Hamiltonian of the chemical unit cell. In that case, the Hamiltonian is multiplied by exp(ikr) with r is the distance between the central atom and the neighbor considered. The obtained metrix is then Bloch diagonal and can be diagonalized for each k-point. It is possible to obtain also the Eigenvectors for each of the eigenvalues.

In this particular case, the size of the unit cell must be 1 1 1 i.e. Nsize 1 1 1

The input looks like this:

• gra_ft_Ham calculate the FT of the Hamiltonian
• calc_evec calculate the Eigenvectors
• q_N_highsym 4 number of high symmetry path

Then, the high symmetry points along the path are given

q_highs_pts
0.0 0.0 0.0     1 Gamma
0.333 0.333 0.0 1 R
0.5 0.0 0.0     1 M
0 0 0           1  Gamma

• q_highs_dist 0.002 to obtain a uniform path, the distance between the 2 successive k-points is given.

Fourier transform of the effective field

to come