Block correlated coupled cluster method with a complete-active-space self-consistent-field reference function: the...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate description of bond breaking and dissociation processes is crucial in chemistry.
• Existing methods like internally contracted multireference configuration interaction (IC-MRCI) and complete active space second-order perturbation theory (CASPT2) have limitations.

Purpose of the Study:

• To develop and implement a new theoretical framework, the block correlated coupled cluster (BCCC) theory, for describing chemical bond dissociation.
• To specifically formulate and test the four-block correlation level of BCCC (CAS-BCCC4) using a complete active space self-consistent field (CASSCF) reference function.

Main Methods:

• Development of the spin orbital formulation for the CAS-BCCC4 approach, ensuring invariance to unitary transformations.
• Implementation of the CAS-BCCC4 method.
• Application to study potential energy surfaces for bond breaking in C2, N2, and simultaneous double bond dissociation in H2O.

Main Results:

• The CAS-BCCC4 approach accurately describes the entire dissociation process for the studied systems (C2, N2, H2O).
• Performance of CAS-BCCC4 is superior to IC-MRCI and CASPT2 for these dissociation processes.
• CAS-BCCC4 exhibits a relatively small size-extensivity error, particularly for N2.

Conclusions:

• The developed CAS-BCCC4 method provides a highly accurate and robust tool for studying chemical bond dissociation.
• CAS-BCCC4 offers a promising alternative to existing methods, demonstrating improved accuracy and favorable error characteristics.