An equations-of-motion approach to quantum mechanics: application to a   model phase transition
  We present a generalized equations-of-motion method that efficiently calculates energy spectra and matrix elements for algebraic models. The method is applied to a 5-dimensional quartic oscillator that exhibits a quantum phase transition between vibrational and rotational phases. For certain parameters, 10 by 10 matrices give better results than obtained by diagonalising 1000 by 1000 matrices. 
S.Y. Ho, G. Rosensteel, and D.J. Rowe