Approximate Force Constants from Uncoupled Self-Consistent Field Perturbation Theory Using Nonhybrid Density...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Nuclear Hessians are crucial for molecular properties and optimizations.
• Calculating Hessians scales steeply with system size due to coupled-perturbed SCF (CP-SCF).

Purpose of the Study:

• To evaluate the performance of the uncoupled SCF (UC-SCF) approximation for calculating nuclear Hessians.
• To assess UC-SCF's accuracy for vibrational frequencies and normal modes in pure DFT.

Main Methods:

• Comparison of UC-SCF and CP-SCF methods.
• Calculations using various exchange-correlation functionals (including Hartree-Fock) and basis sets.
• Application to organic and organometallic molecules.

Main Results:

• UC-SCF performs poorly for Hartree-Fock and hybrid DFT but remarkably well for pure DFT.
• UC-SCF accurately predicts vibrational frequencies and normal modes, even for transition states.
• UC-SCF shows good agreement with coupled calculations for pure DFT functionals.

Conclusions:

• UC-SCF is a viable and efficient approximation for calculating vibrational properties in pure DFT.
• UC-SCF can be used for thermodynamic property calculations and challenging geometry optimizations.