Bond stretching mimics different levels of electron correlation and provides
a challenging testbed for approximate many-body computational methods. Using
the recently developed phaseless auxiliary-field quantum Monte Carlo (AF QMC)
method, we examine bond stretching in the well-studied molecules BH and N2​,
and in the H50​ chain. To control the sign/phase problem, the phaseless AF
QMC method constrains the paths in the auxiliary-field path integrals with an
approximate phase condition that depends on a trial wave function. With single
Slater determinants from unrestricted Hartree-Fock (UHF) as trial wave
function, the phaseless AF QMC method generally gives better overall accuracy
and a more uniform behavior than the coupled cluster CCSD(T) method in mapping
the potential-energy curve. In both BH and N2​, we also study the use of
multiple-determinant trial wave functions from multi-configuration
self-consistent-field (MCSCF) calculations. The increase in computational cost
versus the gain in statistical and systematic accuracy are examined. With such
trial wave functions, excellent results are obtained across the entire region
between equilibrium and the dissociation limit.Comment: 8 pages, 3 figures and 3 tables. Submitted to JC