Bond breaking with auxiliary-field quantum Monte Carlo

摘要

Bond stretching mimics different levels of electron correlation and providesa challenging testbed for approximate many-body computational methods. Usingthe recently developed phaseless auxiliary-field quantum Monte Carlo (AF QMC)method, we examine bond stretching in the well-studied molecules BH and N$_2$,and in the H$_{50}$ chain. To control the sign/phase problem, the phaseless AFQMC method constrains the paths in the auxiliary-field path integrals with anapproximate phase condition that depends on a trial wave function. With singleSlater determinants from unrestricted Hartree-Fock (UHF) as trial wavefunction, the phaseless AF QMC method generally gives better overall accuracyand a more uniform behavior than the coupled cluster CCSD(T) method in mappingthe potential-energy curve. In both BH and N$_2$, we also study the use ofmultiple-determinant trial wave functions from multi-configurationself-consistent-field (MCSCF) calculations. The increase in computational costversus the gain in statistical and systematic accuracy are examined. With suchtrial wave functions, excellent results are obtained across the entire regionbetween equilibrium and the dissociation limit.