Kylin 1.0: An Ab-Initio Density Matrix Renormalization Group Quantum Chemistry Program

The accurate evaluation of electron correlations is highly necessary for the proper descriptions of the electronic structures in strongly correlated molecules, ranging from bond-dissociating molecules, polyradicals, to large conjugated molecules and transition metal complexes. For this purpose, in this paper, a new ab-initio quantum chemistry program Kylin 1.0 for electron correlation calculations at various quantum many-body levels, including configuration interaction (CI), perturbation theory (PT), and density matrix renormalization group (DMRG), is presented. In addtion, fundamental quantum chemical methods such as Hartree-Fock self-consistent field (HF-SCF) and the complete active space SCF (CASSCF) are aslo implemented. The Kylin 1.0 program possesses these features: (1) efficient DMRG implementation based on the matrix product operator (MPO) formulation for describing static electron correlation within a large active space composed of more than 100 orbitals, supporting both $\rm U(1)_{n} \times U(1)_{S_z}$ and $\rm U(1)_{n} \times SU(2)_{S}$ symmetries; (2) efficient second-order DMRG-self-consistent field (SCF) implementation; (3) externally-contracted multi-reference CI (MRCI) and Epstein-Nesbet PT with DMRG reference wave functions for including the remaining dynamic electron correlation outside the large active spaces. In this paper, we introduce the capabilities and numerical benchmark examples of the Kylin 1.0 program.