Set the bounds for ansatz parameters in VQE

Question

How can I make intervals for free parameters when I use VQE function? (https://qiskit.org/documentation/stubs/qiskit.algorithms.VQE.html)

I have built a parametrized ($\theta_1,...,\theta_n$) circuit without using ansatz in Qiskit Circuit Library and I would like to set e.g. $\theta_i \in \left[0,\pi\right]$.

Answer 1

You can set the attribute parameters_bounds of a circuit to the desired intervals like below:

from qiskit import QuantumCircuit
from qiskit.circuit import Parameter
a=Parameter('a')
b=Parameter('b')
ansatz=QuantumCircuit(2)
ansatz.ry(a,0)
ansatz.ry(b,1)
ansatz.parameter_bounds=[[0,np.pi]]*2 

Then you can run your vqe program.