  We report an efficient implementation of a second-order multireference
perturbation theory based on the driven similarity renormalization group
(DSRG-MRPT2) [C. Li and F. A. Evangelista, J. Chem. Theory Comput. 11, 2097
(2015)]. Our implementation employs factorized two-electron integrals to avoid
storage of large four-index intermediates. It also exploits the block structure
of the reference density matrices to reduce the computational cost to that of
second-order M{\o}ller$-$Plesset perturbation theory. Our new DSRG-MRPT2
implementation is benchmarked on ten naphthyne isomers using basis sets up to
quintuple-$\zeta$ quality. We find that the singlet-triplet splittings
($\Delta_\text{ST}$) of the naphthyne isomers strongly depend on the
equilibrium structures. For a consistent set of geometries, the
$\Delta_\text{ST}$ values predicted by the DSRG-MRPT2 are in good agreements
with those computed by the reduced multireference coupled cluster theory with
singles, doubles, and perturbative triples.
