Restricted active space spin-flip configuration interaction approach: theory, implementation and examples.

Area of Science:

• Quantum chemistry
• Computational chemistry

Background:

• The spin-flip (SF) method is a computational technique used to study electronic states.
• Accurate description of electronic states, especially nearly degenerate ones, is crucial in chemistry.
• Existing methods may face limitations in balancing accuracy and computational cost.

Purpose of the Study:

• To present a new formulation of the spin-flip (SF) method.
• To investigate its performance in describing various chemical phenomena.
• To assess its computational efficiency and accuracy.

Main Methods:

• The new method is based on restricted active space (RAS) principles.
• It defines the electronic wave function using an active space and alpha-to-beta excitations from a Hartree-Fock reference.
• A specific truncation of the wave function, including hole and particle contributions, ensures a linear dependence of amplitudes on molecular size for a fixed active space.

Main Results:

• The method was applied to study bond-breaking, singlet-triplet gaps in linear acenes, electronic transitions in Ni(II) complexes, and radical states.
• It successfully investigated the ground state multiplicity of 28 non-Kekulé structures.
• The approach demonstrated a well-balanced description of nearly degenerate electronic states.

Conclusions:

• The presented spin-flip (SF) method provides a robust and accurate approach for describing challenging electronic structures.
• It offers a favorable balance between accuracy and computational cost.
• This method is suitable for investigating a wide range of chemical systems, including those with complex electronic properties.