Multi-Reference Epstein-Nesbet Perturbation Theory with Density Matrix Renormalization Group Reference Wavefunction

Abstract: 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).