Methods for computing nuclear magnetic resonance chemical shielding in large systems. Multiple cluster and charge field approaches

Ab initio calculations show that additivity of the intermolecular shielding exists in a model system consisting of fluorobenzene interacting with hydrogen fluoride molecules, C₆H₅F(HF)_n, where n=1-5. These results indicate that it should be possible to perform chemical shielding calculations on large system by dividing it into a series of smaller clusters. For M atoms divided into M/N clusters of N atoms, the time savings for large M is on the order of M³/16N³, a time savings of ≈ 60 for M= 100, N = 10. We demonstrate the feasibility of using point charges to model long-range electrostatic field effects on shielding by comparing the results of full ab initio calculations with those obtained by using point charges to represent the HF molecules in the C₆H₅F-(HF)_n clusters. This comparison shows generally good agreement between the two approaches so long as the point charges are > 2.5 Å from all the atoms in the molecule to which the nucleus belongs, a situation which should pertain for many macromolecules. Addition of 1000 point charges to the C₆H₅F system increased computational time by only 50% and appears to offer promise for investigations of chemical shielding in proteins and nucleic acids, where both short-large (electronic) and longer-range (electrostatic field) effects may be important.