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A systematic improvement to UGA-SSMRCCSD equations and its implication for potential energy curves
Dibyajyoti Chakravarti1, Sangita Sen2, Debashis Mukherjee1
1Centre for Quantum Engineering, Research, and Education (CQuERE), TCG CREST, Kolkata, India.
The Journal of Chemical Physics
|October 3, 2023
Summary
The Unitary Group Adaptation (UGA) provides an efficient spin adaptation strategy. A new, more rigorous formulation of UGA-state-specific multireference coupled cluster theory systematically improves accuracy for electronic structure calculations.
Area of Science:
- Quantum Chemistry
- Computational Many-Body Theory
Background:
- The Unitary Group Adaptation (UGA) offers a compact spin adaptation strategy for spin-free Hamiltonians.
- State-specific (SS) Jeziorski-Monkhorst Ansatz based multireference coupled cluster (MRCC) theory faces challenges with non-commuting spin-free cluster operators.
Purpose of the Study:
- To present a more rigorous formulation of UGA-SSMRCC theory.
- To enable systematic approximations for emergent terms in the Bloch equation.
- To compare theoretical and numerical performance against a previous formulation.
Main Methods:
- Utilizing a normal ordered exponential parametrization of the wave operator to handle non-commutativity.
- Exact factorization of unlinked terms in the Bloch equation.
- Theoretical analysis and numerical demonstration of potential energy curves.
Main Results:
- The new formulation allows for a hierarchy of systematic approximations.
- Previous UGA-SSMRCC formulation relied on non-systematic sufficiency conditions.
- The contribution of newly identified terms gains importance with increased active orbitals.
Conclusions:
- The rigorous UGA-SSMRCC formulation provides a systematically improvable approach.
- The new terms significantly impact potential energy curves.
- This advancement is crucial for accurate electronic structure calculations in complex systems.
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