Higher-order equation-of-motion coupled-cluster methods for electron attachment

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Electron attachment and excitation energies are crucial molecular properties.
• Standard computational methods struggle with molecules exhibiting multideterminant wave functions.
• High-order coupled-cluster methods are computationally demanding but offer high accuracy.

Purpose of the Study:

• To implement and apply high-order equation-of-motion coupled-cluster (EA-EOM-CC) methods for electron attachment.
• To assess the accuracy of these methods for electron affinities and excitation energies of open-shell molecules.
• To investigate the impact of including higher-order electron attachment and cluster excitation operators.

Main Methods:

• Implementation of EA-EOM-CC methods using the TCE symbolic algebra program.
• Application of size-extensive truncations for electron attachment (2p-1h, 3p-2h, 4p-3h) and cluster excitation (doubles, triples, quadruples) operators.
• Extrapolation techniques used to remove basis set errors for calculated properties.

Main Results:

• Standard EA-EOM-CCSD methods show errors exceeding 2 eV for molecules with multideterminant wave functions.
• Incorporating higher-order operators (e.g., 3p-2h attachment, triple excitations) significantly improves accuracy.
• The advanced EA-EOM-CC methods predict electron affinities and excitation energies within 0.01 eV of experimental values.

Conclusions:

• High-order EA-EOM-CC methods are essential for accurate calculations of electron attachment and excitation energies in challenging systems.
• The inclusion of 3p-2h electron attachment and triple cluster excitation operators is particularly important for achieving high accuracy.
• These developed methods provide a reliable tool for studying electron affinities and excitation energies of open-shell molecules.