Solvable coupled cluster states in $d=\infty$ with prescribed momentum distribution

Two of the most successful ideas in many-body theory, the exponential coupled cluster ansatz and DMFT, have to date developed separately. We establish a connection between them, by showing that the infinite dimensional limit of the hypercubic lattice allows the exact closed-form evaluation of static expectation values of coupled cluster wave functions, provided the cluster amplitudes admit a low-rank decomposition. As a result, evaluation errors -- a serious concern in conventional coupled cluster theory -- can be disentangled from limitations to expressivity of coupled cluster states. For this family of coupled cluster states, the cavity construction in $d\to\infty$ exactly reduces the environment of a lattice site to a Gaussian Grassmann kernel, and irreducible diagrams for one-body expectation values are found to be site-diagonal (in analogy to the DMFT self-energy). The resulting ``impurity problem" is an integral over a finite number of Grassmann variables rather than a dynamical quantum impurity problem, so that static observables derive exactly from a static self-consistency condition: a wave function counterpart of DMFT. For the simplest case of a cluster amplitude restricted to rank-one doubles, we obtain the momentum distribution in closed form. We show that any momentum distribution can be realized by a state that is the exact and unique ground state of an explicit parent Hamiltonian with a gapless single-particle tunnelling spectrum. By construction these coupled cluster states variationally recover low order perturbation theory for the Hubbard model. Their failure in the strong coupling regime near the Mott transition diagnoses a fundamental limitation of coupled cluster doubles. Intriguingly, our family of coupled cluster wave functions includes a gapless state with zero quasiparticle weight, with a momentum distribution that is completely smooth at the Fermi energy.

Source: arXiv:2610.41976v1 - http://arxiv.org/abs/2610.41976v1 PDF: https://arxiv.org/pdf/2610.41976v1 Original Link: http://arxiv.org/abs/2610.41976v1