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Showing content from https://github.com/qutip/qutip/issues/2669 below:

Strange behavior with list-based time-dependent Hamilitonian and mesolve · Issue #2669 · qutip/qutip · GitHub

When I add a zero matrix with time dependence (even if the time dependence is constant) my Hamiltonian, it changes the way mesolve behaves.
Am I misunderstanding the way H is constructed?

from qutip import *

N = 4
a = destroy(N)
psi0 = fock(N, 0)
omega = 1
kappa = 1

def coeff_1(t, args): 
    return 1

def coeff_2(t, args):
    return 1

H0 = omega * a.dag() * a
H1 = a.dag() + a
H2 = 0 * (a.dag() + a)

coeff_test1 = coefficient(coeff_1)
coeff_test2 = coefficient(coeff_2)

H = QobjEvo([H0, [H1, coeff_test1], H2])
H_prime = QobjEvo([H0, [H1, coeff_test1], [H2, coeff_test2]])

times = np.linspace(0, 1, 1000)

result1 = mesolve(H, psi0, times, [kappa * a], [], args={})
result2 = mesolve(H_prime, psi0, times, [kappa * a], [], args={})

n1 = expect(a.dag() * a, result1.states[-1])
n2 = expect(a.dag() * a, result2.states[-1])

print('diff: ' + str(n1 - n2))
assert n1 == n2

diff: -1.2532650784482082
Traceback (most recent call last):
  File "/test.py", line 50, in <module>
    assert n1 == n2
           ^^^^^^^^
AssertionError

Adding time-dependence to a zero component of H shouldn't change anything.

QuTiP Version:      5.0.2
Numpy Version:      1.26.4
Scipy Version:      1.13.1
Cython Version:     None
Matplotlib Version: 3.9.0
Python Version:     3.12.4
Number of CPUs:     12
BLAS Info:          Generic
INTEL MKL Ext:      False
Platform Info:      Darwin (arm64)
Installation path:  /Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/site-packages/qutip

No response

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