  The phase diagram of two-level boson Hamiltonians, including the Interacting
Boson Model (IBM), is studied beyond the standard mean field approximation
using the Holstein-Primakoff mapping. The limitations of the usual intrinsic
state (mean field) formalism concerning finite-size effects are pointed out.
The analytic results are compared to numerics obtained from exact
diagonalizations. Excitation energies and occupation numbers are studied in
different model space regions (Casten triangle for IBM) and especially at the
critical points.
