  We have implemented recently developed multiple-projector pseudopotentials
into the planewave based auxiliary-field quantum Monte Carlo (pw-AFQMC) method.
Multiple-projector pseudopotentials can yield smaller planewave cut-offs while
maintaining or improving transferability. This reduces the computational cost
of pw-AFQMC, increasing its reach to larger and more complicated systems. We
discuss the use of non-local pseudopotentials in the separable
Kleinman-Bylander form, and the implementation in pw-AFQMC of the
multiple-projector optimized norm-conserving pseudopotential ONCVPSP of Hamann.
The accuracy of the method is first demonstrated by equation-of-state
calculations of the ionic insulator NaCl and more strongly correlated metal Cu.
The method is then applied to calibrate the accuracy of density functional
theory (DFT) predictions of the phase stability of recently discovered high
temperature and pressure superconducting sulfur hydride systems. We find that
DFT results are in good agreement with pw-AFQMC, due to near cancellation of
electron-electron correlation effects between different structures.
