Iterative universal state selective correction for the Brillouin-Wigner multireference coupled-cluster theory.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Previous a posteriori Universal State-Selective (USS) corrections were introduced.
• These corrections aim to improve the accuracy of quantum chemical calculations.

Purpose of the Study:

• To develop an iterative form of the USS correction.
• To formulate USS corrections using left Bloch equations.
• To investigate the convergence of USS corrections towards the full configuration interaction (FCI) limit.

Main Methods:

• An iterative USS correction method was developed by correcting effective Hamiltonian matrix elements.
• USS corrections were formulated via the left Bloch equations.
• Numerical assessments were performed on model systems, ozone, and tetramethyleneethane molecules at singles and doubles and perturbative triple levels.

Main Results:

• The iterative USS correction was shown to be a viable alternative to the a posteriori Brillouin-Wigner coupled cluster size-extensivity correction.
• The iterative USS correction is insensitive to intruder states.
• The method performs well in cases where the a posteriori method fails, such as for the asymmetric vibration mode of ozone.

Conclusions:

• The iterative USS correction offers a robust and accurate method for quantum chemistry.
• This approach overcomes limitations of previous methods, particularly concerning intruder states and specific molecular vibrations.