Pruning the Hamiltonian Matrix in MULTIMODE: Test for C2H4 and Application to CH3NO2 Using a New Ab Initio Potential...

Area of Science:

• Computational Chemistry
• Molecular Physics
• Quantum Chemistry

Background:

• Accurate prediction of molecular vibrational energies is crucial for understanding chemical reactions and properties.
• Nitromethane (CH3NO2) presents a computational challenge due to its complex vibrational modes, particularly the internal methyl torsion.

Purpose of the Study:

• To develop and apply a new full-dimensional potential energy surface (PES) for nitromethane.
• To compute vibrational energies of nitromethane using the MULTIMODE code and a novel pruning scheme.
• To validate the computational approach against experimental data.

Main Methods:

• A precise, permutationally invariant PES was generated using 17,049 electronic energies calculated with CCSD(T)-F12b/HaDZ.
• Vibrational self-consistent field/virtual state configuration interaction (VSCF/VCI) calculations were performed using the MULTIMODE code.
• A pruning scheme was implemented to manage large Hamiltonian matrices in 14-mode calculations, excluding the challenging torsional mode.

Main Results:

• The new PES accurately describes the vibrational degrees of freedom of nitromethane, including the methyl torsion.
• The 14-mode vibrational calculations, utilizing the pruning scheme, yielded results in very good agreement with experimental fundamental frequencies.
• Diffusion Monte Carlo calculations confirmed the nearly free-rotor nature of the torsional motion.

Conclusions:

• The developed PES and computational methodology provide accurate vibrational energies for nitromethane.
• The pruning scheme is an effective strategy for handling complex vibrational calculations.
• The study enhances the understanding of nitromethane's vibrational dynamics and provides a benchmark for future theoretical investigations.