If I change the value of a magnetic field in my problem, the program suddenly starts to output error messages of the type:
MAGNEW,TXYZ: 0.31640638004497
U,V,W -6.56290015166051E-04 9.07632797085983E-03 0.56242650150838
In the card MGNFIELD one usually defines the components of the magnetic field, but if they are all set = 0, a user subroutine MAGFLD is called, with the following arguments: ( X, Y, Z, BTX, BTY, BTZ, B, NREG, IDISC ). Here, BTX, BTY and BTZ are not the components, but the direction cosines! The absolute value is given by B. Looking at the direction cosines printed in the error message, it is clear that U, V and W (the direction cosines) are not properly normalised. The sum of their squares is 0.316406, as shown by the TXYZ value).