The correlation-consistent composite approach: application to the G3/99 test set.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The correlation-consistent composite approach (ccCA) is an ab initio composite technique.
• Previous versions have shown success in reproducing experimental data for various systems.
• The G3/99 test set comprises 223 enthalpies of formation, 88 adiabatic ionization potentials, 58 adiabatic electron affinities, and 8 adiabatic proton affinities.

Purpose of the Study:

• To apply the improved ccCA formalism to the G3/99 test set.
• To evaluate the accuracy and performance of ccCA compared to existing computational methods.
• To assess the method's reliability for calculating thermochemical properties of atoms and molecules.

Main Methods:

• The study employed an improved ccCA formalism, incorporating coupled cluster computations, scalar relativistic corrections, and optimized basis sets.
• Complete basis set extrapolation of MP2 energies was utilized.
• The method was applied to a comprehensive set of thermochemical data within the G3/99 test set.

Main Results:

• The ccCA method achieved a near-zero mean deviation (-0.10 kcal mol(-1)) and a mean absolute deviation of 0.96 kcal mol(-1) for the G3/99 set.
• The accuracy achieved is comparable to the established G3X model chemistry.
• ccCA demonstrated performance equal to or exceeding Gn methods for first-row atoms and showed improvement for second-row atoms.

Conclusions:

• The enhanced ccCA formalism provides highly accurate atomic and molecular energies without empirical parameters.
• ccCA represents a significant advancement in computational chemistry for thermochemical property prediction.
• The method offers a reliable and accurate alternative to existing Gn model chemistries, particularly for systems involving second-row elements.