entropy_vn for three particles

[sperelat]

Hello.

I am doing a simulation on atom-photon interaction and I tried entropy_vn() to check the correlation between particles and modes.

I assumed that if an atom and two light modes are in a pure state, von neumann entorpy of the state would be zero, but the number I got

was negative number.

Foe example, when I tried

max_n=25

mode1=2.3

mode2=2.3

psi0 = tensor(basis(4,0), coherent(max_n, mode1),coherent(max_n,mode2))             

, the von Neumann entropy of psi0

entropy_vn(psi0)

was  around -660125.

It seems that this problem happens only when the number of particles and modes are larger than 3.

I wonder how   I can fix this problem.