ATOMIC-2 Protocol for Thermochemistry.

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Thermochemistry

Background:

• The ATOMIC protocol utilizes bond separation reactions (BSRs) to reduce computational cost for calculating atomization energies and enthalpies of formation.
• Existing composite models balance accuracy and computational expense, approximating bond separation energies at the complete-basis-set limit.

Purpose of the Study:

• To introduce ATOMIC-2, an improved version of the thermochemistry protocol.
• To enhance accuracy and computational efficiency through refined methods and an integrated error and uncertainty model (ATOMIC-2um).

Main Methods:

• Geometry optimization and zero-point-energy calculations are performed using density functional theory (PBE0-D3/6-311G(d)).
• Improved complete-basis-set (CBS) extrapolations towards the Full CI limit are used for auxiliary molecule atomization energies.
• An error and uncertainty model (ATOMIC-2um) is developed to estimate bias and uncertainty in energy contributions.

Main Results:

• ATOMIC-2 demonstrates significant computational savings and improved accuracy compared to previous methods.
• The bias-corrected ATOMIC-2um protocol shows higher accuracy than ATOMIC-1 and Gaussian-4 theory, with a consistent uncertainty model.
• The framework is extended to include challenging bond topologies, considering neutral, closed-shell molecules with H, C, N, O, and F atoms.

Conclusions:

• ATOMIC-2, particularly with the ATOMIC-2um uncertainty model, offers a more accurate and reliable approach to thermochemical calculations.
• The protocol addresses limitations of previous methods by incorporating advanced computational techniques and robust error estimation.
• The developed computational code is made available for performing ATOMIC-2um analyses.