Model protein excited states: MRCI calculations with large active spaces vs CC2 method

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurately calculating excited states in protein models is crucial for understanding their function.
• The standard complete active space self-consistent field (CASSCF) method struggles with large systems due to computational cost.
• Developing efficient alternatives to CASSCF is essential for studying larger, more complex molecular systems.

Purpose of the Study:

• To benchmark alternative computational methods for excited states in phenylalanine protein models.
• To assess methods that reduce computational cost while maintaining accuracy compared to CASSCF.
• To evaluate the performance of generalized active space self-consistent field (GASSCF), occupation-restricted multiple active space (ORMAS), and coupled cluster (CC2) methods for larger systems.

Main Methods:

• Benchmarking calculations on excited states of phenylalanine protein models.
• Comparison of CASSCF with reduced-determinant strategies: restricted active space self-consistent field, GASSCF, and ORMAS.
• Inclusion of dynamic correlation using complete active space second-order perturbation theory and multireference difference dedicated configuration interaction.
• Assessment of approximate second-order coupled cluster (CC2) for larger systems.

Main Results:

• GASSCF and ORMAS schemes efficiently reduce the configuration interaction expansion without sacrificing accuracy for singlet ππ* and nπ*CO excited states.
• These methods maintain the accuracy of excitation energies and the nature of excited states compared to CASSCF.
• The CC2 method demonstrates accurate performance for large systems, comparable to more computationally intensive methods.

Conclusions:

• GASSCF and ORMAS are highly efficient strategies for extending accurate excited-state calculations beyond the CASSCF limit.
• Guidelines for optimal application of these schemes to large systems are proposed.
• The CC2 method shows significant potential for accurately treating excited states in very large molecular systems.