  We report a scalable Fortran implementation of the phaseless auxiliary-field
quantum Monte Carlo (ph-AFQMC) and demonstrate its excellent performance and
beneficial scaling with respect to system size. Furthermore, we investigate
modifications of the phaseless approximation that can help to reduce the
overcorrelation problems common to the ph-AFQMC. We apply the method to the 26
molecules in the HEAT set, the benzene molecule, and water clusters. We observe
a mean absolute deviation of the total energy of 1.15 kcal/mol for the
molecules in the HEAT set; close to chemical accuracy. For the benzene
molecule, the modified algorithm despite using a single-Slater-determinant
trial wavefunction yields the same accuracy as the original phaseless scheme
with 400 Slater determinants. Despite these improvements, we find systematic
errors for the CN, CO$_2$, and O$_2$ molecules that need to be addressed with
more accurate trial wavefunctions. For water clusters, we find that the
ph-AFQMC yields excellent binding energies that differ from CCSD(T) by
typically less than 0.5 kcal/mol.
