Externally-Contracted Multireference Configuration Interaction Method Using a DMRG Reference Wave Function.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Strongly Correlated Systems

Background:

• Density matrix renormalization group (DMRG) efficiently handles static correlation in large active spaces.
• Dynamic correlation is crucial for strongly correlated systems but computationally expensive with traditional methods.
• Existing DMRG-MRCI methods face limitations with large active spaces and basis sets due to high computational costs.

Purpose of the Study:

• To develop a more scalable and efficient DMRG-MRCI method for quantum chemistry.
• To overcome the computational bottlenecks associated with high-order RDMs and MRCI Hamiltonian dimensions.
• To enable accurate electronic structure calculations for larger and more complex molecular systems.

Main Methods:

• Implemented a novel DMRG-MRCI approach using reconstructed CASCI-type configurations from DMRG wave functions.
• Employed the entropy-driving genetic algorithm (EDGA) for configuration reconstruction.
• Integrated DMRG with MRCI using an external contraction (ec) scheme, bypassing internal contraction limitations.

Main Results:

• The new DMRG-ec-MRCI method effectively bypasses the computation of high-order RDMs.
• The number of MRCI configurations is independent of the virtual orbital count, reducing computational cost.
• Demonstrated the method's capability on benchmark systems like Cr2, acenes, and a lanthanide complex.

Conclusions:

• The proposed DMRG-ec-MRCI method significantly enhances the applicability of accurate quantum chemical calculations.
• It allows for larger active spaces (beyond 30 orbitals) and larger basis sets.
• This advancement is crucial for studying complex, strongly correlated molecules with unprecedented accuracy.