Combining Strongly Orthogonal Geminals with Explicitly Correlated Corrections

Area of Science:

• Quantum chemistry
• Computational physics
• Theoretical chemistry

Background:

• Pair coupled cluster doubles (pCCD) with explicit correlation (F12) shows promise for describing electron correlation.
• Antisymmetrized product of interacting two-electron fragments (geminals) is a key concept in pCCD.
• Strongly orthogonal geminals (SLG) offer an efficient way to capture static correlation.

Purpose of the Study:

• To explore pair-function based methodologies combined with explicit correlation.
• To develop and apply a new computational model for electron correlation.
• To investigate the electronic structure of multireference systems.

Main Methods:

• Employing strongly orthogonal geminals (SLG) in the wave function Ansatz.
• Correcting the SLG wave function with an explicitly correlated F12 model via perturbation theory.
• Using Slater-type functions as correlation factors and exploring different amplitude determination schemes (Hylleraas-functional, s- and p-wave cusp conditions).

Main Results:

• The SLG-F12 model effectively describes dynamic correlation missing in SLG by incorporating double excitations.
• Explicitly correlated terms are internally contracted and solely responsible for dynamic correlation.
• The model is applied to study singlet-triplet state splitting in small biradicals and multireference systems.

Conclusions:

• The SLG-F12 model provides an efficient and accurate method for capturing both static and dynamic electron correlation.
• This approach is suitable for investigating complex electronic structures in multireference systems.
• The combination of SLG and F12 methods offers a promising direction for quantum chemical calculations.