High-accuracy extrapolated ab initio thermochemistry. III. Additional improvements and overview

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Quantum Chemistry

Background:

• The High-Accuracy Extrapolated Ab Initio Thermochemistry (HEAT) model chemistry is a leading method for calculating molecular atomization energies.
• Basis-set size and correlated treatments of the diagonal Born-Oppenheimer approximation are crucial factors influencing computational accuracy.
• Achieving chemical accuracy (<1 kJ mol(-1)) requires careful consideration of electron correlation effects, particularly core-valence separation.

Purpose of the Study:

• To evaluate the impact of increased basis-set size on HEAT model chemistry.
• To assess the effect of correlated treatments of the diagonal Born-Oppenheimer approximation within the HEAT framework.
• To investigate the limitations of approximate core-valence electron correlation treatments in high-accuracy thermochemistry.

Main Methods:

• Utilizing the HEAT theoretical model chemistry framework.
• Performing calculations with systematically increased basis-set sizes.
• Incorporating correlated treatments for the diagonal Born-Oppenheimer approximation.
• Analyzing the separation of core-valence electron correlation effects.

Main Results:

• Increasing basis-set size and adding correlated diagonal Born-Oppenheimer treatments showed minimal improvement in HEAT's accuracy for molecular atomization energies.
• The overall HEAT strategy's accuracy was found to be higher than many individual components due to fortuitous cancellation of errors.
• Approximate additive treatments for core-valence electron correlation were found to have significant limitations for achieving sub-kJ/mol accuracy.

Conclusions:

• The HEAT model chemistry's accuracy for atomization energies is robust, with marginal gains from basis-set augmentation and diagonal Born-Oppenheimer correlation.
• Fortuitous error cancellation plays a significant role in the overall effectiveness of the HEAT approach.
• Precise theoretical thermochemistry (<1 kJ mol(-1)) necessitates advanced treatments beyond simple additive core-valence correlation approximations.