Challenging compounds for calculating hyperpolarizabilities: p-quinodimethane derivatives.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Electronic Structure Theory

Background:

• Accurate calculation of molecular electronic properties, such as hyperpolarizabilities, is crucial for understanding material responses to electric fields.
• Electron correlation effects significantly influence electric field response properties, necessitating sophisticated theoretical approaches.
• Previous studies highlight the challenges in accurately predicting these properties for systems with diradical character.

Purpose of the Study:

• To evaluate the accuracy of various wave function and density functional theory (DFT) methods for calculating the hyperpolarizabilities of p-quinodimethane derivatives.
• To compare computational results against benchmark values obtained using the coupled cluster method with single and double excitations and perturbative triples [CCSD(T)].
• To assess the impact of basis set choice on the calculated hyperpolarizabilities.

Main Methods:

• High-level wave function methods, including coupled cluster (CCSD, CCSD(T)) and Møller-Plesset perturbation theory (MP2, UMP3, UMP4) with and without spin projection.
• Density functional theory (DFT) employing a wide array of exchange-correlation functionals.
• Systematic evaluation of basis set effects on the calculated electronic properties.

Main Results:

• The coupled cluster with single and double excitations (CCSD) method demonstrated the closest agreement with the CCSD(T) reference values for second hyperpolarizability.
• MP2 theory performed adequately for closed-shell compounds but showed limitations for open-shell systems.
• Spin-projected UMP3 and UMP4 methods provided good agreement for neutral p-quinodimethane but less so for charged species. DFT methods, without spin projection, generally yielded less accurate results for these systems.

Conclusions:

• For accurate determination of second hyperpolarizability in low-diradical character p-quinodimethane derivatives, the CCSD method is recommended.
• The choice of theoretical method, particularly accounting for electron correlation and spin states, is critical for reliable predictions.
• Further investigation into spin-projection effects and DFT functional performance is warranted for charged and open-shell systems.