  The accurate electronic structure calculation for strongly correlated
chemical systems requires an adequate description for both static and dynamic
electron correlation, and is a persistent challenge for quantum chemistry. In
order to account for static and dynamic electron correlations accurately and
efficiently, in this work we propose a new method by integrating the density
matrix renormalization group (DMRG) method and multi-reference second-order
Epstein-Nesbet perturbation theory (ENPT2) with a selected configuration
interaction (SCI) approximation. Compared with previous DMRG-based dynamic
correlation methods, the DMRG-ENPT2 method extends the range of applicability,
allowing us to efficiently calculate systems with very large active space
beyond 30 orbitals. We demonstrate this by performing calculations on H$_2$S
with an active space of (16e, 15o), hexacene with an active space of (26e, 26o)
and 2D H$_{64}$ square lattice with an active space of (42e, 42o).
