  The phaseless auxiliary-field quantum Monte Carlo (AF QMC) method [S. Zhang
and H. Krakauer, Phys. Rev. Lett. 90, 136401 (2003)] is used to carry out a
systematic study of the dissociation and ionization energies of second-row
group 3A-7A atoms and dimers, Al, Si, P, S, Cl. In addition, the P_2 dimer is
compared to the third-row As_2 dimer, which is also triply-bonded. This method
projects the many-body ground state by means of importance-sampled random walks
in the space of Slater determinants. The Monte Carlo phase problem, due to the
electron-electron Coulomb interaction, is controlled via the phaseless
approximation, with a trial wave function |Psi_T>. As in previous calculations,
a mean-field single Slater determinant is used as |Psi_T>. The method is
formulated in the Hilbert space defined by any chosen one-particle basis. The
present calculations use a planewave basis under periodic boundary conditions
with norm-conserving pseudopotentials. Computational details of the planewave
AF QMC method are presented. The isolated systems chosen here allow a
systematic study of the various algorithmic issues. We show the accuracy of the
planewave method and discuss its convergence with respect to parameters such as
the supercell size and planewave cutoff. The use of standard norm-conserving
pseudopotentials in the many-body AF QMC framework is examined.
