  The VB-QMC method is presented in this chapter. It consists of using in
quantum Monte Carlo (QMC) approaches with a wave function expressed as a
usually short expansion of classical Valence-Bond (VB) structures supplemented
by a Jastrow factor to account for dynamical correlation. Two variants exist:
the VB-VMC (using variational Monte Carlo) and VB-DMC (using diffusion Monte
Carlo) methods. QMC algorithms circumvent the notorious non-orthogonality issue
of classical VB approaches, and allow highly efficient calculations on
massively parallel machines. Calculation of VB weights and resonance energies
are possible at the VB-VMC level, which makes VB-VMC a correlated method
retaining all the interpretative capabilities of classical VB methods. Several
recent applications are shown to illustrate the potential of this method as a
modern alternative to classical VB methods to study ground and excited states
of molecules.
