The present version of CAIN accepts a constant external field only.
The covariant form of the equation of motion
where is the proper time and the electromagnetic field tensor, can be solved exactly when the field is constant. The eigenvalues of the matrix is given by and , where
Then, the solution is
where the upper (lower) sign applies to () and with being the antisymmetric tensor of rank 4.
The classical spin motion of electrons is given by the Thomas-BMT
where a is the coefficient of anomalous magnetic moment and
Figure 1: Field dependence of the anomalous magnetic moment of electron
When the field is very strong, a is different from the well-known
value but is a function of the field strength
characterized the parameter .
The functional form of is shown in Fig.1. Simple polynomial approximations are used in CAIN.