  The Full Configuration Interaction Quantum Monte Carlo (FCIQMC) method has
proved able to provide near-exact solutions to the electronic Schr\"odinger
equation within a finite orbital basis set, without relying on an expansion
about a reference state. However, a drawback to the approach is that being
based on an expansion of Slater determinants, the FCIQMC method suffers from a
basis set incompleteness error that decays very slowly with the size of the
employed single particle basis. The FCIQMC results obtained in a small basis
set can be improved significantly with explicitly correlated techniques. Here,
we present a study that assesses and compares two contrasting `universal'
explicitly correlated approaches that fit into the FCIQMC framework; the
$[2]_{R12}$ method of Valeev {\em et al.}, and the explicitly correlated
canonical transcorrelation approach of Yanai {\em et al}. The former is an {\em
a posteriori} internally-contracted perturbative approach, while the latter
transforms the Hamiltonian prior to the FCIQMC simulation. These comparisons
are made across the 55 molecules of the G1 standard set. We found that both
methods consistently reduce the basis set incompleteness, for accurate
atomization energies in small basis sets, reducing the error from 28~mE$_{\text
h}$ to 3-4~mE$_{\text h}$. While many of the conclusions hold in general for
any combination of multireference approaches with these methodologies, we also
consider FCIQMC-specific advantages of each approach.
