Multilevel CC2 and CCSD Methods with Correlated Natural Transition Orbitals

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multilevel coupled cluster (CC) methods treat active orbital spaces at higher accuracy than inactive spaces.
• Existing methods require iterative calculations for inactive orbital amplitudes.

Purpose of the Study:

• Introduce the multilevel CC2 method using CC2 for the active space.
• Present a simplified multilevel CCSD method with iterative CC2 amplitudes for the inactive space.
• Apply correlated natural transition orbitals for active space determination.

Main Methods:

• Developed multilevel CC2 and simplified multilevel CCSD formulations.
• Employed correlated natural transition orbitals to define active orbital spaces.
• Performed proof-of-concept calculations for valence excitation energies.

Main Results:

• Established convergence of multilevel CC2 and CCSD valence excitation energies.
• Successfully applied the methods to larger systems: p-nitroaniline in water and amoxicillin.
• Demonstrated the utility for molecules in solution and charge transfer excitations.

Conclusions:

• The developed multilevel CC methods offer accurate and efficient electronic structure calculations.
• These methods are valuable for studying complex molecular systems and phenomena like charge transfer.
• The simplified multilevel CCSD formulation reduces computational cost for inactive space calculations.