Theoretical Study on the Properties of Small Molecules with the Block Effective Hamiltonian Theory and Multireference...

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Effective Hamiltonian theory provides a framework for simplifying complex quantum systems.
• Accurate calculation of excitation energies and electron affinities is crucial for understanding molecular properties.

Purpose of the Study:

• To extend the effective Hamiltonian theory to n-blocks.
• To calculate vertical and adiabatic excitation energies for H2O, NH2, and CH2.
• To investigate the effect of basis sets on electron affinities and compare various theoretical models.

Main Methods:

• Extension of effective Hamiltonian theory to four and n-blocks.
• Calculation of excitation energies using three-block-effective Hamiltonian theory (BEHT3), multireference perturbation theories, and multireference configuration interaction (MRCI).
• Investigation of electron affinities using complete active space self-consistent field (CASSCF), Møller-Plesset perturbation theory (MP2, MP3), coupled-cluster methods (CCSD, CCSD(T)), and BEHT3.

Main Results:

• BEHT3 energies for ground and excited states of H2O, NH2, and CH2 converge from below to MRCI values.
• BEHT3 excitation energies converge from above to experimental or higher-level theoretical results.
• Diffuse basis functions are critical for accurate electron affinity calculations.
• BEHT3 demonstrates superior performance compared to other considered theoretical models.

Conclusions:

• The n-block effective Hamiltonian theory is a robust extension of the original framework.
• BEHT3 offers a reliable and accurate method for calculating molecular excitation energies and electron affinities.
• The choice of basis set, particularly diffuse functions, significantly impacts electron affinity predictions.