gutzwiller approximation bose Hubbard model References Square bose Hubbard model https://doi.org/10.1103/PhysRevA.77.015602 (QMC) https://doi.org/10.1103/PhysRevB.59.12184 (series expansion) Gutzwiller approximation https://doi.org/10.1209/0295-5075/22/4/004