Relativistic internally contracted multireference electron correlation methods.

Area of Science:

• Quantum chemistry
• Computational physics
• Electronic structure theory

Background:

• Simulating relativistic and quasi-degenerate electronic structures of molecules containing heavy elements presents significant computational challenges.
• Accurate theoretical methods are crucial for understanding the properties of such systems.

Purpose of the Study:

• To develop and implement advanced computational methods for accurate electronic structure calculations of molecules with transition-metal and heavy elements.
• To demonstrate the accuracy of these methods through simulations of rovibrational transition energies and absorption spectra.

Main Methods:

• Internally contracted relativistic multireference configuration interaction (ic-MRCI) and complete active space second-order perturbation (CASPT2) methods based on the four-component Dirac Hamiltonian.
• An automatic code generator for translating second-quantized ansätze to tensor-based equations and efficient computer code for ic-MRCI and CASPT2.
• Strongly contracted n-electron valence state perturbation theory (NEVPT2) derived and implemented manually.

Main Results:

• Accurate simulations of relativistic, quasi-degenerate electronic structure are enabled by the implemented methods.
• Rovibrational transition energies and absorption spectra for HI and TlH were successfully computed.
• The presented methods demonstrate high accuracy for the studied molecular systems.

Conclusions:

• The developed ic-MRCI and CASPT2 methods, along with NEVPT2, provide accurate tools for electronic structure calculations of heavy-element molecules.
• These computational advancements are vital for advancing research in quantum chemistry and materials science.
• The successful application to HI and TlH validates the utility of these methods for complex molecular systems.