Dynamical effects in ab initio NMR calculations: classical force fields fitted to quantum forces.

Area of Science:

• Computational chemistry
• Solid-state physics
• Nuclear Magnetic Resonance (NMR) spectroscopy

Background:

• NMR chemical shifts are sensitive to molecular structure.
• Ab initio molecular dynamics (MD) can capture these effects but is computationally expensive.
• Accurate MD simulations require consistent methods for structure generation and shift calculation.

Purpose of the Study:

• To develop a computationally feasible method for calculating NMR chemical shifts in molecular crystals, including dynamical effects.
• To bridge the gap between high-accuracy ab initio calculations and the long timescales needed for MD simulations.
• To investigate the impact of finite size effects on NMR chemical shifts in L-alanine crystals.

Main Methods:

• Fitting classical force fields to ab initio forces derived from density functional theory (DFT).
• Generating structural ensembles using classical MD simulations based on the fitted force fields.
• Calculating NMR chemical shifts for various structural configurations.

Main Results:

• The developed methodology accurately reproduces NMR chemical shifts obtained from expensive ab initio MD.
• Classical force fields fitted to ab initio forces enable efficient MD simulations for NMR shift calculations.
• Finite size effects in supercell calculations were found to be significant for dynamics.

Conclusions:

• A hybrid approach combining classical MD with ab initio-derived force fields provides a computationally efficient route to study NMR chemical shifts in molecular crystals.
• This method allows for the inclusion of anharmonic dynamical effects crucial for accurate chemical shift prediction.
• Careful consideration of system size is necessary for reliable dynamic simulations and NMR shift calculations.