A zeroth-order active-space frozen-orbital embedding scheme for multireference calculations.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Large-scale quantum chemistry computations are often limited by high computational expense.
• Accurate modeling of multireference systems requires advanced theoretical methods.
• Efficient embedding schemes are crucial for studying complex chemical environments.

Purpose of the Study:

• To develop a simple and automatic active space embedding theory, termed ASET(0).
• To enable the use of any multireference dynamical correlation method with frozen-orbital environment treatment.
• To assess the accuracy and applicability of ASET(0) for various chemical systems.

Main Methods:

• Developed zeroth-order active space embedding theory (ASET(0)).
• Integrated ASET(0) with second-order multireference driven similarity renormalization group (MR-DSRG).
• Applied the combined method to benchmark calculations (excitation energies, bond breaking) and surface adsorption studies.

Main Results:

• ASET(0) combined with MR-DSRG accurately computed excitation energies for 1-octene.
• The method successfully modeled bond-breaking processes in ethane and pentyldiazene.
• Singlet-triplet gaps of diradicals on a NaCl surface were effectively studied using ASET(0).

Conclusions:

• ASET(0) is a powerful and accurate embedding scheme for multireference systems.
• The method is particularly effective in the weak to medium coupling regime between the fragment and environment.
• ASET(0) offers a computationally efficient approach for large-scale chemical system studies.