Quantum-computing within a bosonic context: Assessing finite basis effects on prototypical vibrational Hamiltonian spectra

Abstract: Quantum computing has recently been emerging in theoretical chemistry as a realistic avenue meant to offer computational speedup to challenging eigenproblems in the context of strongly-correlated molecular systems or extended materials. Most studies so far have been devoted to the quantum treatment of electronic structure and only a few were directed to the quantum treatment of vibrational structure, which at the moment remains not devoid of unknowns. In particular, we address here a formal problem that arises when simulating a vibrational model under harmonic second quantization, whereby the disruption of the closure relation (resolution of the identity) -- which occurs when truncating the infinite bosonic basis set -- may have some serious effects as regards the correct evaluation of Hamiltonian matrix elements. This relates intimately to the normal ordering of products of ladder operators. In addition, we discuss the relevance of choosing an adequate primitive basis set within the present context with respect to its variational convergence properties. Such fundamental, yet consequential, aspects are illustrated numerically in the present work on a one-dimensional anharmonic Hamiltonian model corresponding to a double-well potential showing strong tunneling, of interest both for vibrational spectroscopy and chemical reactivity.