We demonstrate with soluble models how to employ the effective Hamiltonian approach of Lee and Suzuki to obtain all the exact eigenvalues of the full Hamiltonian. We propose a new iteration scheme to obtain the effective Hamiltonian and demonstrate its convergence properties.

The various approximations used in the construction of a first-principles effective Hamiltonian for BaTiO3, and their effects on the calculated transition temperatures, are discussed. An effective Hamiltonian for BaTiO3 is constructed not from first-principles calculations, but from the structural energetics of an atomistic shell model for BaTiO3 of Tinte et al. This allows the elimination of c...

Embedding calculations that find approximate solutions to the Schrödinger equation for large molecules and realistic solids are performed commonly in a three step procedure involving (i) construction of a model system with effective interactions approximating the low energy physics of the initial realistic system, (ii) mapping the model system onto an impurity Hamiltonian, and (iii) solving the...

Alkali-metal rare-earth chalcogenide $AREC{h}_{2}$ (A = alkali or monovalent metal, RE rare earth, Ch O, S, Se, Te) is a large family of quantum spin liquid candidates we discovered recently. Unlike ${\mathrm{YbMgGaO}}_{4}$, most members in the family, except for oxide ones, have relatively small crystalline electric field (CEF) energy gap from ground state to first excited state. This makes co...