Performance of density functional theory in computing nonresonant vibrational (hyper)polarizabilities

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Vibrational hyperpolarizabilities are crucial for nonlinear optical properties.
• Polymethineimine (PMI) oligomers are model systems for studying these properties.
• Accurate theoretical prediction of these properties requires reliable computational methods.

Purpose of the Study:

• To assess the performance of various exchange-correlation functionals in predicting static and dynamic nonresonant vibrational (hyper)polarizabilities.
• To evaluate these functionals for all-trans polymethineimine (PMI) oligomers up to eight monomer units.
• To identify the most suitable functional for these calculations.

Main Methods:

• Calculations of vibrational (hyper)polarizabilities using BLYP, PBE0, B3LYP, BHandHLYP, CAM-B3LYP, LC-BLYP, and HSE functionals.
• Comparison of functional results against reference values from Møller-Plesset second-order perturbation theory (MP2) and coupled cluster methods (CCSD, CCSD(T)).
• Assessment of the Pople 6-31+G(d) basis set reliability for PMI oligomers.

Main Results:

• CAM-B3LYP demonstrated the best performance across several properties, including static linear polarizability and second hyperpolarizability.
• No single functional accurately predicted all vibrational (hyper)polarizabilities.
• Hartree-Fock and all tested functionals yielded incorrect signs for electric field-induced second harmonic generation.
• The Pople 6-31+G(d) basis set was found to be unreliable due to predicting nonplanar geometries.

Conclusions:

• CAM-B3LYP is a promising functional for predicting vibrational hyperpolarizabilities in PMI oligomers, but further refinement is needed.
• Careful selection of both functionals and basis sets is essential for accurate predictions.
• The study highlights limitations in current computational methods for certain nonlinear optical properties.