PASPT2: A Size-Extensive and Size-Consistent Partial-Active-Space Multistate Multireference Second-Order Perturbation...

Area of Science:

• Quantum chemistry
• Computational physics
• Electronic structure theory

Background:

• Strongly correlated electron systems present significant challenges for traditional electronic structure methods.
• Existing methods like intermediate normalization-based general-model-space state-universal coupled-cluster theory (IN-GMS-SU-CCSD) suffer from disconnected terms, impacting accuracy.
• Accurate modeling of these systems is crucial for understanding chemical reactions and material properties.

Purpose of the Study:

• To formulate a novel partial-active-space (PAS) multistate (MS) multireference second-order perturbation theory (MRPT2), termed PASPT2.
• To address and overcome the issue of disconnected terms present in the parent IN-GMS-SU-CCSD theory.
• To develop a size-extensive and size-consistent method for electronic structure calculations of strongly correlated systems.

Main Methods:

• Linearization of IN-GMS-SU-CCSD theory.
• Formulation of PASPT2 by employing a reference-specific zeroth-order Hamiltonian.
• Ensuring the effective/intermediate Hamiltonian is connected and closed for diagonalized energies.
• Utilizing a partial active space (PAS) construction for supermolecules as a direct product of fragments.

Main Results:

• PASPT2 successfully avoids disconnected terms in its amplitude equations.
• The effective/intermediate Hamiltonian in PASPT2 is connected and closed, leading to fully connected energies.
• PASPT2 demonstrates strict size-extensivity, a significant improvement over IN-GMS-SU-CCSD.
• The method is also size-consistent when the PAS is defined appropriately for molecular fragments.

Conclusions:

• PASPT2 provides a robust and accurate theoretical framework for studying strongly correlated electron systems.
• The method's size-extensivity and size-consistency enhance its reliability for diverse chemical applications.
• Demonstrated efficacy on prototypical systems validates PASPT2 as a powerful tool in computational chemistry.