It is possible to use PW codes for QMC calculations of atoms and molecules.
The procedure is the same as for the solid-state calculations but certain conditions have to be met to ensure the boundary conditions are properly handled.
It is important to converge DFT calculations with respect to the cell size. Use stricter convergence criteria than those employed for DFT calculations. Not only the energy, the orbitals have to be converged.
The supercell is used to define the basic grid for the bspline orbitals which operate on [0,1)x[0,1)x[0,1).
The moves to the region where the wavefunctions are not defined are rejected.