Q-Chem Webinar 70: Calculating NMR Shielding with GIAO

July 31st, 2023

Q-Chem is pleased to present our 70th Q-Chem Webinar (presented on July 28, 2023 by Jonathan Wong), now available for viewing here.

Jonathan Wong standing in front of a monument and mountain on a city street.

Molecular property calculations have long been a cornerstone of computational chemistry, providing invaluable insights for experimental work. Among these properties, nuclear magnetic resonance (NMR) shieldings play a crucial role in characterizing chemical environments. Conventionally, these calculations rely on the often tedious derivation and implementation of the analytical form of the energy derivative. On the other hand, the energy derivative can be carried out in a numerical manner. In this webinar, Jonathan Wong will present a method-independent, fully numerical finite difference approach for NMR shielding calculations, utilizing gauge-including atomic orbitals (GIAO), that is recently implemented in Q-Chem. This effectively leverages the abundant quantum chemistry methods in Q-Chem, without the need for analytical derivative implementation.

In the first part, Jonathan will introduce the theory underlying our Q-Chem implementation of the GIAO calculations, including the new matrix elements required, fully complex SCF and post-SCF calculations, and finally the numerical derivative. In the second part, he will present proof-of-concept results with κ-regularized MP2 and MP2.X, where a variable fraction X of third-order correlation (MP3) is incorporated. Results show significant improvements with κ-MP2 for 13C and 15N shieldings. Surprisingly, MP2.6 outperforms CCSD for all heavy nuclei, indicating potential renormalization of double amplitudes to account for neglected triple substitutions. Lastly, he will use a few examples to demonstrate how to use this new module in Q-Chem, as well as exciting ongoing developments.