Permanent variational wave functions for bosons

We study the performance of permanent states (the bosonic counterpart of the Slater determinant state) as approximating functions for bosons, with the intention to develop variational methods based upon them. For a system of N identical bosons, a permanent state is constructed by taking a set of N arbitrary (not necessarily orthonormal) single-particle orbitals, forming their product and then symmetrizing it. It is found that for the one-dimensional Bose-Hubbard model with the periodic boundary condition and at unit filling, the exact ground state can be very well approximated by a permanent state, in that the permanent state has high overlap (at least 0.96 even for 12 particles and 12 sites) with the exact ground state and can reproduce both the ground state energy and the single-particle correlators to high precision. For a generic model, we have devised a greedy algorithm to find the optimal set of single-particle orbitals to minimize the variational energy or maximize the overlap with a target state. It turns out that quite often the ground state of a bosonic system can be well approximated by a permanent state by all the criteria of energy, overlap, and correlation functions. And even if the error is apparent, it can often be remedied by including more configurations, i.e., by allowing the variational wave function to be a combination of multiple permanent states. The algorithm is used to study the stability of a two-particle system, with great success. All these suggest that permanent states are very effective as variational wave functions for bosonic systems, and hence deserve further studies. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

We study the performance of permanent states as approximating functions for bosons, with the aim of developing variational methods based on them. We find that permanent states can well approximate the exact ground state for the one-dimensional Bose-Hubbard model. For a generic model, a greedy algorithm is devised to find the optimal set of single-particle orbitals. It is found that permanent states are effective variational wave functions for bosonic systems.