Perturbation theory in the complete degenerate active space (CDAS-PT2)

Area of Science:

• Quantum chemistry
• Theoretical chemistry
• Computational physics

Background:

• Multireference perturbation theory (MRPT) with a one-electron zeroth-order Hamiltonian is computationally efficient for excited-state calculations.
• A key limitation of current MRPT methods is the use of small model spaces, which can compromise accuracy.

Purpose of the Study:

• To propose and formulate an extended MRPT method utilizing a complete active space.
• To enhance the accuracy of excited-state calculations by overcoming the limitations of small model spaces.

Main Methods:

• Formulation of a one-electron zeroth-order Hamiltonian suitable for a complete active space.
• Application of the proposed MRPT method to model systems including LiF, ethylene, and trans-butadiene.

Main Results:

• The proposed MRPT method demonstrated superior accuracy compared to XMCQDPT2 in most tested cases.
• The improvement in accuracy was particularly notable when employing small active spaces.

Conclusions:

• The extended MRPT method with a complete active space offers a significant advancement in the accurate description of excited states.
• This approach provides a more robust and accurate alternative to existing methods, especially in challenging computational scenarios.