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We present phaseless auxiliary-field (AF) quantum Monte Carlo (QMC)calculations of the ground states of some hydrogen-bonded systems. Thesesystems were selected to test and benchmark different aspects of the newphaseless AF QMC method. They include the transition state of H+H$_2$ near theequilibrium geometry and in the van der Walls limit, as well as H$_2$O, OH, andH$_2$O$_2$ molecules. Most of these systems present significant challenges fortraditional independent-particle electronic structure approaches, and many alsohave exact results available. The phaseless AF QMC method is used either with aplanewave basis with pseudopotentials or with all-electron Gaussian basis sets.For some systems, calculations are done with both to compare and characterizethe performance of AF QMC under different basis sets and differentHubbard-Stratonovich decompositions. Excellent results are obtained using asinput single Slater determinant wave functions taken from independent-particlecalculations. Comparisons of the Gaussian based AF QMC results with exact fullconfiguration show that the errors from controlling the phase problem with thephaseless approximation are small. At the large basis-size limit, the AF QMCresults using both types of basis sets are in good agreement with each otherand with experimental values.Comment: 9 pages and 2 figures. submitted to Journal of Chemical Physic

A study of H+H2 and several H-bonded molecules by phaseless auxiliary-field quantum Monte Carlo with plane wave and Gaussian basis sets