Green chemistry meets theoretical chemistry – comparison of 24 quantum chemical methods for calculating NMR shielding constants using the RGB model

The idea of green chemistry has already found many recipients, but not yet among computational chemists. Although theoretical methods indeed do not use chemical reagents and do not produce laboratory waste, due to the complexity of calculations they often require a lot of energy and can generate a significant carbon footprint. In our opinion, the basic question is not “Should we take into account the carbon footprint of computational methods in their assessment?”, but “How to do it properly?” In this work, we have attempted to address this issue by using an RGB model, a well-known tool used for assessing analytical chemistry methods, that has been adapted to the specifics of computational chemistry (RGB_in-silico). The model uses three primary parameters: the calculation error (red), the carbon footprint resulting from the energy consumption of the computer (green), and the computation time (blue). In phase I, acceptability thresholds are adopted. Methods that are unacceptable in at least one aspect are rejected. In phase II methods are comprehensively compared in terms of “whiteness”. The model was validated on 24 quantum chemical methods for calculating NMR shielding constants, differing in functionals and basis sets. The obtained results indicate a huge discrepancy between the methods, and therefore, the need to use a dedicated tool for selecting the optimal one in a rational way. The large values of the carbon footprint clearly indicate that some computational methods cannot be considered as “green in nature”. The RGB_in-silico model seems to be a simple and useful metric.