Assume $|i\rangle,i\in[1,m]$ is a set of states, like the Fock states for Fermions. The question is how to compute the quantity,$\langle i |H|j\rangle,i,j\in[1,m]$, in qiskit. The $H$ here is the qubit form Hamiltonian of some molecule obtained by the Jordan-Wigner transformation.
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You're looking for the matrix elements of $H$ in the basis $|i⟩$. Depending on what $|i⟩$ is, you can probably do so without qiskit. (Indeed, getting the matrix elements of $H$ is a starting point for chemistry calculations with qiskit!) – jecado Oct 19 '21 at 16:19
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As mentioned in a comment, there are many times where you might think you want to do this but actually there may be a more direct classical approach (eg. doing a single-body basis transformation of your Hamiltonian's coefficients).
But, sometimes you might need to do this quantum-ly. This paper discusses one way, which requires twice as many qubits but is otherwise pretty efficient: https://arxiv.org/abs/1909.09114
Essentially you prepare |i⟩ and |j⟩ on two different registers and perform a modified Hadamard test.
jecado
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