  The potential energy curve of the F$_2$ molecule is calculated with
Fixed-Node Diffusion Monte Carlo (FN-DMC) using Configuration Interaction
(CI)-type trial wavefunctions. To keep the number of determinants reasonable
(the first and second derivatives of the trial wavefunction need to be
calculated at each step of FN-DMC), the CI expansion is restricted to those
determinants that contribute the most to the total energy. The selection of the
determinants is made using the so-called CIPSI approach (Configuration
Interaction using a Perturbative Selection made Iteratively). Quite remarkably,
the nodes of CIPSI wavefunctions are found to be systematically improved when
increasing the number of selected determinants. To reduce the non-parallelism
error of the potential energy curve a scheme based on the use of a
$R$-dependent number of determinants is introduced. Numerical results show that
improved FN-DMC energy curves for the F$_2$ molecule are obtained when
employing CIPSI trial wavefunctions. Using the Dunning's cc-pVDZ basis set the
FN-DMC energy curve is of a quality similar to that obtained with FCI/cc-pVQZ.
A key advantage of using selected CI in FN-DMC is the possibility of improving
nodes in a systematic and automatic way without resorting to a preliminary
multi-parameter stochastic optimization of the trial wavefunction performed at
the Variational Monte Carlo level as usually done in FN-DMC.
