An extension of the connected moments expansion to nonlinear Hamiltonians is presented. The new method enables construction of rapidly convergent series for the ground-state energy which requires a relatively small amount of numerical effort.
The t-expansion method for calculating ground-state energies of Hamiltonian systems is generalized to excited states. We introduce quantities Mn(t) and Dn(t) which for $t\ensuremath{\rightarrow}\ensuremath{\infty}$ converge to ${\ensuremath{\Sigma}}_{n}={E}_{0}+{E}_{1}+\dots{}+{E}_{n}$ and ${\ensuremath{\Delta}}_{n}={E}_{n+1}\ensuremath{-}{E}_{n}$, respectively. Like in the standard t-expansion, the t-series for Mn(t) and Dn(t) are constructed from moments of the Hamiltonian. The values of ${\ensuremath{\Sigma}}_{n}$ and ${\ensuremath{\Delta}}_{n}$ can then be estimated using (e.g.) a generalization of the connected-moment expansion of Cioslowski.
The expression for the exact ground-state energy as given by Horn and Weinstein [Phys. Rev. D 30, 1256 (1984)] is approximated using a probability distribution function having a \ensuremath{\delta} function and a uniform distribution. The expression obtained up to second order does not have the singularity that one encounters in the connected-moment expansion. It also gives the same kind of expression for the ground-state energy that recently has been given by Mancini, Zhou, and Meier [Int. J. Quant. Chem. 50, 101 (1994)] when the third moment becomes negative.
This work presents a simple formalism which proposes an estimate of the ground-state energy from a single reference function. It is based on a perturbative expansion but leads to nonlinear coupled equations. It can be viewed as well as a modified coupled cluster formulation. Applied to a series of spin lattices governed by model Hamiltonians the method leads to simple analytic solutions. The so-calculated cohesive energies are surprisingly accurate. Two examples illustrate its applicability to locate phase transition.