SDSPT2s:SDSPT2 with Selection.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Static-dynamic-static (SDS) configuration interaction (CI) is a minimal multireference configuration interaction (MRCI) method.
• SDSPT2 is a CI-like multireference second-order perturbation theory (PT2) that approximates SDSCI.
• SDSPT2 handles single and multiple roots uniformly, allowing for configuration selection.

Purpose of the Study:

• To present an efficient approximation to SDSPT2, termed SDSPT2s (SDSPT2 with selection).
• To enable accurate quantum chemical calculations for larger and more complex systems.
• To demonstrate the applicability of SDSPT2s through challenging computational showcases.

Main Methods:

• Configuration selection is applied to a large complete active space (CAS) to reduce the reference space.
• The interacting space is reduced by connecting the reduced reference space to a small portion of the full first-order interacting space.
• The most computationally expensive part of the reduced interacting space is truncated by bypassing its generation and applying an integral-based cutoff.
• Internal contraction coefficients are evaluated and stored efficiently.

Main Results:

• The selection-truncation procedure in SDSPT2s achieves this reduction with marginal loss of accuracy.
• SDSPT2s is shown to be applicable to systems that are intractable for the standard CAS-based SDSPT2.
• Several challenging computational examples demonstrate the effectiveness of the SDSPT2s method.

Conclusions:

• SDSPT2s significantly enhances the applicability of multireference second-order perturbation theory to larger quantum chemical systems.
• The method provides a computationally feasible approach for studying complex molecular systems previously beyond reach.
• This work advances the capabilities of theoretical chemistry for accurate electronic structure calculations.