State specific equation of motion coupled cluster method in general active space.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The state specific equation of motion coupled cluster (SS-EOMCC) method is a powerful multireference approach for electronic structure calculations.
• Previous applications were limited to systems with small active spaces, such as biradicals and single bond breaking.

Purpose of the Study:

• To extend the applicability of the SS-EOMCC method to systems with general active spaces.
• To assess the accuracy of the extended SS-EOMCC method for various molecular systems.

Main Methods:

• The study utilizes an internally contracted multireference approach.
• It involves optimizing orbitals and dynamical correlation for the target state.
• Diagonalization of the transformed Hamiltonian in a multireference configuration-interaction singles space accounts for nondynamical correlation and orbital relaxation.

Main Results:

• The SS-EOMCC method was successfully extended to handle general active spaces.
• Accuracy was evaluated through studies on F(2), H(2)O, CO, and N(2) molecules.
• The convergence strategy for these calculations was analyzed in detail.

Conclusions:

• The extended SS-EOMCC method provides a qualitatively correct and rigorously spin-adapted wave function.
• The method is invariant to orbital rotations and requires only one- and two-particle density matrices.
• This advancement significantly broadens the scope of systems amenable to accurate SS-EOMCC treatment.