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We report a scalable Fortran implementation of the phaseless auxiliary-fieldquantum Monte Carlo (ph-AFQMC) and demonstrate its excellent performance andbeneficial scaling with respect to system size. Furthermore, we investigatemodifications of the phaseless approximation that can help to reduce theovercorrelation problems common to the ph-AFQMC. We apply the method to the 26molecules in the HEAT set, the benzene molecule, and water clusters. We observea mean absolute deviation of the total energy of 1.15 kcal/mol for themolecules in the HEAT set; close to chemical accuracy. For the benzenemolecule, the modified algorithm despite using a single-Slater-determinanttrial wavefunction yields the same accuracy as the original phaseless schemewith 400 Slater determinants. Despite these improvements, we find systematicerrors for the CN, CO$_2$, and O$_2$ molecules that need to be addressed withmore accurate trial wavefunctions. For water clusters, we find that theph-AFQMC yields excellent binding energies that differ from CCSD(T) bytypically less than 0.5 kcal/mol.

Benchmark Phaseless Auxiliary-Field Quantum Monte Carlo Method for Small Molecules