  We explore the use in quantum Monte Carlo (QMC) of trial wave functions
consisting of a Jastrow factor multiplied by a truncated
configuration-interaction (CI) expansion in Slater determinants obtained from a
CI perturbatively selected iteratively (CIPSI) calculation. In the CIPSI
algorithm, the CI expansion is iteratively enlarged by selecting the best
determinants using perturbation theory, which provides an optimal and automatic
way of constructing truncated CI expansions approaching the full CI limit. We
perform a systematic study of variational Monte Carlo (VMC) and fixed-node
diffusion Monte Carlo (DMC) total energies of first-row atoms from B to Ne with
different levels of optimization of the parameters (Jastrow parameters,
coefficients of the determinants, and orbital parameters) in these trial wave
functions. The results show that the reoptimization of the coefficients of the
determinants in VMC (together with the Jastrow factor) leads to an important
lowering of both VMC and DMC total energies, and to their monotonic convergence
with the number of determinants. In addition, we show that the reoptimization
of the orbitals is also important in both VMC and DMC for the Be atom when
using a large basis set. These reoptimized Jastrow-CIPSI wave functions appear
as promising, systematically improvable trial wave functions for QMC
calculations.
