A spin-flip variant of the second-order approximate coupled-cluster singles and doubles method

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled-cluster methods are vital for accurate electronic structure.
• Spin-flip methods are crucial for studying diradicals and magnetic properties.
• Approximate methods are needed for computational efficiency.

Purpose of the Study:

• Implement and evaluate a spin-flip variant of the CC2 method.
• Assess its performance for potential energy curves and spin-state splittings.
• Compare its accuracy against other theoretical methods like ADC(2).

Main Methods:

• Spin-flip variant of the second-order approximate coupled-cluster singles and doubles (CC2) method.
• Utilized resolution-of-the-identity or Cholesky decomposition for electron repulsion integrals.
• Calculated potential energy curves (H2, HF) and singlet-triplet splittings (diradicals, copper complexes).

Main Results:

• The spin-flip CC2 method shows performance comparable to spin-flip ADC(2) for diradicals.
• Spin-flip CC2 incorrectly predicts a barrierless dissociation for ozone.
• Spin-conserving CC2 also fails for ozone's dissociation surface.

Conclusions:

• Spin-flip CC2 is a viable method for certain electronic structure problems, particularly diradicals.
• The method exhibits limitations, failing to accurately describe ozone's dissociation.
• Spin-flip ADC(2) and coupled-cluster singles and doubles provide more reliable results for ozone.