A magnetic field
can be applied to stabilize for example an antiferromagnetic phase. The magnetic field acts only on the spin, AND ALTERS THE ENERGY AND WAVEFUNCTIONS!! This is done with an entry like:
magnetic_field_kvec 1 1 0 1
The last number switches the field on (=1) or off (=0), the first 3 INTEGER numbers specify the wave number in reciprocal lattice coordinates.
The amplitude (normalized such that 
Ryd) is entered as:
magnetic_field_strength 0.001