Extending spin-symmetry projected coupled-cluster to large model spaces using an iterative null-space projection...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Spin-extended coupled-cluster (ECC) methods offer accurate electronic structure descriptions.
• The nonorthogonal and overcomplete parametrization in ECC necessitates orthogonalization.
• Existing methods for orthogonalization are computationally infeasible for large systems due to metric size.

Purpose of the Study:

• To address the computational bottleneck in ECC calculations.
• To enable the application of ECC to larger and more complex molecular systems.
• To improve the accuracy and applicability of quantum chemical methods.

Main Methods:

• Introduced an orbital-invariant approximate coupled-cluster (CC) method within the spin-projection manifold.
• Developed an iterative projection technique to remove linear dependencies without metric diagonalization.
• Applied the method to study the complete basis set limit of F2 and transition metal complexes.

Main Results:

• The iterative projection method avoids computationally expensive metric diagonalization.
• The approach significantly expands the applicability of ECC to larger systems.
• Accurate calculations were performed for challenging systems like F2, NiO, Mn2, and [Cu2O2]2+.

Conclusions:

• The developed method overcomes the limitations of previous ECC approaches.
• This advancement allows for more accurate and feasible electronic structure calculations.
• Potential issues with spin-projected Hartree-Fock molecular orbitals were identified and solutions proposed.