Multi-state local complete active space second-order perturbation theory using pair natural orbitals (PNO-MS-CASPT2).

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of electronic excitation energies is crucial for understanding molecular properties.
• Traditional methods face computational challenges with increasing system size and number of electronic states.

Purpose of the Study:

• To develop a computationally efficient and scalable multistate complete active space second-order perturbation theory (CASPT2) method.
• To enable accurate calculation of excitation energies for large molecular systems.

Main Methods:

• Implementation of a multistate CASPT2 method using pair natural orbitals and projected atomic orbitals for the virtual space.
• Achieving linear scaling of computational effort with inactive orbitals.
• Testing approximations to the CASPT2 zeroth-order Hamiltonian to reduce computational cost for excited states.

Main Results:

• The developed method achieves linear scaling, enabling calculations on systems with up to 400 correlated electrons and 45 electronic states.
• Local approximations in the virtual space were found to have a negligible impact on excitation energies.
• Proposed approximations to the zeroth-order Hamiltonian significantly reduce computational effort with minimal impact on accuracy.

Conclusions:

• The new multistate CASPT2 method provides an efficient and accurate approach for computing excitation energies in large molecules.
• The method's scalability and the effectiveness of proposed approximations open new possibilities for complex quantum chemistry calculations.