Extensions of numerical methods for strongly correlated electron systems

This work presents extensions of the numerical methods for strongly correlated electron systems. The first part of the thesis discusses extensions and applications of the quantum cluster theories to the systems of classical spins. It is shown that such extensions can provide faster convergence through better estimation of the effects of fluctuations, yet they can also possess shortcomings which limit their application in the studies of the phase transitions. The second part of the thesis is dedicated to the numerical studies of the Hubbard model. Present Quantum Monte Carlo methods are reviewed and relationships among them are elucidated. The final part of the thesis contains the application of the developed numerical methods to investigate the phase diagram of the two-dimensional Hubbard model, especially the evidence of the Quantum Critical Point (QCP) at a finite doping. High accuracy results for thermodynamic quantities are presented in support of the existence of the QCP at a finite doping in two-dimensional Hubbard model. The relation of the QCP to the charge fluctuations is revealed and a mechanism that relates QCP to incipient phase separation is proposed.