You must put a dot in all the numbers which are supposed to be floats, so as to be correct while doing division. In gnuplot
2/3 = 0
2.0/3.0 = 0.66666..7
In your example, you are suffering from division by zero, but gnuplot does not give good error message.
Here is corrected version which runs without any error
B0P = 4.0; B0 = 30.0
V0 = 36.0; E0 = -535.0; ...