  Herein, we report accurate atomization energy calculations for 55 molecules
in the Gaussian-2 (G2) set using lattice regularized diffusion Monte Carlo
(LRDMC). We compare the Jastrow-Slater determinant ansatz with a more flexible
JsAGPs (Jastrow correlated antisymmetrized geminal power with singlet
correlation) ansatz. AGPs is built from pairing functions, which explicitly
include pairwise correlations among electrons and hence, this ansatz is
expected to be more efficient in recovering the correlation energy. The AGPs
wave functions are first optimized at the variational Monte Carlo (VMC) level,
which includes both the Jastrow factor and the nodal surface optimization. This
is followed by the LRDMC projection of the ansatz. Remarkably, for many
molecules, the LRDMC atomization energies obtained using the JsAGPs ansatz
reach chemical accuracy ($\sim$1 kcal/mol) and for most other molecules, the
atomization energies are accurate within $\sim$5 kcal/mol. We obtained a mean
absolute deviation of 1.6 kcal/mol with JsAGPs and 3.2 kcal/mol with JDFT
(Jastrow factor + Slater determinant with DFT orbitals) ansatz. This work shows
the effectiveness of the flexible AGPs ansatz for atomization energy
calculations and electronic structure simulations in general.
