  We investigate the optimization of flexible tailored real-space Jastrow
factors for use in the transcorrelated (TC) method in combination with highly
accurate quantum chemistry methods such as initiator full configuration
interaction quantum Monte Carlo (FCIQMC). Jastrow factors obtained by
minimizing the variance of the TC reference energy are found to yield better,
more consistent results than those obtained by minimizing the variational
energy. We compute all-electron atomization energies for the challenging
first-row molecules C2 , CN, N2 , and O2 and find that the TC method yields
chemically accurate results using only the cc-pVTZ basis set, roughly matching
the accuracy of non-TC calculations with the much larger cc-pV5Z basis set. We
also investigate an approximation in which pure three-body excitations are
neglected from the TC-FCIQMC dynamics, saving storage and computational cost,
and show that it affects relative energies negligibly. Our results demonstrate
that the combination of tailored real-space Jastrow factors with the
multi-configurational TC-FCIQMC method provides a route to obtaining chemical
accuracy using modest basis sets, obviating the need for basis-set
extrapolation and composite techniques.
