Accurate Thermochemistry with Multireference Methods: A Stress Test for Internally Contracted Multireference...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• The internally contracted multireference coupled-cluster method with single, double and perturbative triple excitations (icMRCCSD(T)) is a key tool for high-accuracy thermochemistry.
• Existing formulations of icMRCCSD(T) face challenges with specific Hamiltonians and overestimation of perturbative corrections, particularly for systems with open shells.

Purpose of the Study:

• To evaluate and improve the performance of the icMRCCSD(T) method for computational high-accuracy thermochemistry.
• To address identified limitations in the current icMRCCSD(T) formulation, specifically concerning the choice of the zeroth-order Hamiltonian and amplitude equations.

Main Methods:

• Tested the icMRCCSD(T) method against standard single-reference coupled-cluster methods for first-row dinuclear compounds and 3d-transition metal hydrides (MnH, FeH, CoH).
• Investigated the use of the Dyall Hamiltonian versus the effective Fock operator as the zeroth-order Hamiltonian.
• Implemented a modified amplitude equation, termed the '(cT*) correction', to account for terms quadratic in pair clusters, inspired by recent work on single-reference coupled-cluster theory.

Main Results:

• Identified two primary issues with the standard icMRCCSD(T): a biased description from the Dyall Hamiltonian and overestimation of perturbative triple excitations.
• The improved icMRCCSD(cT*)F method, utilizing the effective Fock operator and the (cT*) correction, significantly enhanced accuracy.
• The improved method achieved errors below 2 kJ/mol for total and atomization energies across the tested compounds, outperforming previous benchmarks.

Conclusions:

• The modified icMRCCSD(cT*)F method provides a robust and highly accurate approach for computational thermochemistry, particularly for challenging systems like transition metal compounds.
• The effective Fock operator and the (cT*) correction are crucial for resolving the limitations of the standard icMRCCSD(T) formulation.
• This advancement offers a more reliable tool for theoretical chemists studying molecular energetics.