Downfolding complex materials problems onto model Hamiltonians for quantum computers*

Simulating the properties of quantum materials is expected to be an exciting application for quantum computers, where we hope to see advantages over classical hardware. The complexity of ab initio Hamiltonians describing the physics of application-relevant materials places them beyond the realm of possibility for solution on near-term hardware with a limited number of qubits. Various Hamiltonian approximations, including Hamiltonian downfolding, offer a possibility towards simulating complex materials on near-term hardware. This is accomplished by approximating the relevant physics of a material through their representation by simpler model Hamiltonians, such as the Hubbard Hamiltonian or extensions of it. Here we employ a first-principles methodology for deriving downfolded multi-band extended Hubbard Hamiltonians of materials, capturing strong electronic correlation and electron-phonon coupling, based on the formalism of Wannier functions and the calculation of the screened Coulomb interaction. We demonstrate for a variety of systems that quantum simulation of downfolded Hamiltonians reproduces key properties, thus establishing downfolding as a promising route to achieve near-term simulation of materials on quantum hardware.