The IPEA dilemma in CASPT2.

Area of Science:

• Computational chemistry
• Quantum chemistry
• Spectroscopy

Background:

• Multi-configurational second-order perturbation theory (CASPT2) is widely used for excited-state properties.
• The IPEA shift was introduced to correct systematic errors in dissociation energies, attributed to unbalanced electronic state descriptions.
• It was hypothesized that the IPEA shift could also improve excitation energy calculations.

Purpose of the Study:

• To evaluate the necessity and impact of the IPEA shift for calculating excited states of organic chromophores.
• To determine if the IPEA shift offers a reliable correction for CASPT2 excitation energies.

Main Methods:

• Literature survey of CASPT2 calculations on organic molecules.
• Benchmark full configuration interaction (FCI) calculations on small molecules.
• CASPT2 calculations with varying IPEA shift values on organic chromophores.

Main Results:

• Unmodified CASPT2 negligibly underestimates excitation energies by 0.02 eV (literature survey).
• FCI benchmarks show excited states are underestimated by only 0.05 eV.
• The IPEA shift's magnitude correlates with system size and basis set, leading to overestimation of excitation energies and contradicting its universal applicability.

Conclusions:

• The IPEA shift is not justified for calculating excited states of organic chromophores.
• Unmodified CASPT2 provides accurate excitation energies with minimal underestimation.
• The IPEA shift's system and basis set dependency undermines its role as a universal correction for excited-state calculations.