Running the same function twice leads to different plots

[saad khalid]

Hey everyone:

I'm not quite sure what I'm doing incorrectly here, but I wrote some code which outputs a plot in the end. I start by running:

n =16;
J = 1;
h = 1;
var('T,h');
spinConfig = Tuples([-1,1],n).list()

Then, I run:

def Ham(spins,J,h):
    total = J*sum(spins[x]*spins[x+1] for x in range(0,len(spins)-1)) + -h * sum(spin for spin in spins)
    return total

def Partition(spinConfig,T):
    return sum(e^(-Ham(spins,J,h)/T) for spins in spinConfig)
T = 1
mag = (1/n)*derivative(-T*log(Partition(spinConfig,T)),h)
plot(mag(h),(-10,10))

Now, if I run just this last part a second time, the plot changes! Can anyone tell me what I am doing incorrectly?

Thanks!