A sequential transformation approach to the internally contracted multireference coupled cluster method

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The internally contracted multireference coupled cluster (ic-MRCC) approach is a powerful tool for describing complex electronic structures.
• Existing ic-MRCC methods can be computationally demanding, limiting their application.

Purpose of the Study:

• To develop a new, computationally efficient wave function ansatz for multireference coupled cluster (MRCC) methods.
• To preserve the accuracy of established ic-MRCC approaches while simplifying their formulation.
• To introduce a novel approximation (sqic-MRCCSD[2]) for practical applications.

Main Methods:

• Formulation of the sequential, internally contracted multireference coupled cluster (sqic-MRCC) wave function ansatz.
• Development of the sqic-MRCCSD[2] approximation, incorporating singles and doubles excitations.
• Application and validation of the methods to molecular systems like BeH2, H2O, O3, p-benzyne, and N2.

Main Results:

• The untruncated sqic-MRCCSD scheme provides results nearly identical to the standard ic-MRCCSD method.
• The sqic-MRCCSD[2] approximation shows minimal deviation (<0.2 mE(h)) from the full ic-MRCCSD for BeH2 dissociation.
• sqic-MRCCSD[2] accurately reproduces results for various molecular properties, matching truncated ic-MRCCSD calculations.

Conclusions:

• The sqic-MRCC approach offers a simplified yet accurate alternative to traditional ic-MRCC methods.
• The sqic-MRCCSD[2] approximation provides a computationally advantageous method with high fidelity to more rigorous treatments.
• Formal proof establishes the equivalence between ic-MRCC, sqic-MRCC, and full configuration interaction methods.