Cluster perturbation theory. V. Theoretical foundation for cluster linear target states

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Coupled cluster (CC) theory is a standard method for high-accuracy electronic structure calculations.
• Calculating properties for excited states or states with higher excitations in CC theory can be computationally demanding.
• Previous work established cluster perturbation (CP) theory for CC target states.

Purpose of the Study:

• To extend cluster perturbation (CP) theory to handle cluster linear (CL) target states.
• To develop a method for calculating energies and molecular properties for CL states.
• To assess the accuracy and computational feasibility of the extended CP theory.

Main Methods:

• Linear parametrization of the CL target state, embedding a CC parent state.
• Development of a series of corrections based on the CC parent-state similarity-transformed fluctuation potential.
• Application of the CP theory with CL target states to calculate ground-state energies for HF, N2, and CH2 molecules.

Main Results:

• The CP theory for CL states simplifies calculations compared to standard CC theory, with quadratic amplitude equations.
• The method exhibits weak size-extensivity, with minimal impact on energy and molecular property calculations.
• Fifth-order CP energies for CL states show excellent agreement (<0.0001 hartree deviation) with CCSDT energies for test molecules.

Conclusions:

• The extended CP theory provides a computationally tractable and accurate approach for CL target states.
• This method serves as a viable and attractive alternative to standard CC theory for high-accuracy calculations.
• It is particularly useful for incorporating triple and higher excitations in electronic structure studies.