The gyromagnetic ratio of the electron. This is of course a physical
constant, and the default value is the exact one that you should not
(i) You want to disconnect the anomalous Zeeman term in the Hamiltonian
(then set it to zero; this number only affects that term);
(ii) You are using an effective Hamiltonian, as is the case when
you calculate a 2D electron gas, in which case you have an effective
gyromagnetic factor that depends on the material.