Foundations of the LRESC model for response properties and some applications

Accurate calculations of some response properties, like the NMR spectroscopic parameters, are quite exigent for the theoretical quantum chemistry models together with the computational codes that are written from them. They need to include a very good description of the electronic density in regions close to the nuclei. When heavy‐atom containing systems are studied, those requirements become even higher. Given that relativistic effects must be included in one way or another on the calculation of response properties of heavy‐atoms and heavy‐atom containing molecules, different schemes were developed during the past decades to include them in as good as possible way. There are some four‐component models, which include relativistic effects in a very compact way, although calculations have large time‐consumption; one also needs to deal with new and unusual four‐component operators. There are also two‐component models, which in general may be less accurate, although their application to property calculations on medium‐size and large‐size molecules are feasible, and they maintain the application of usual operators. In this review, we give the fundamentals of the two‐component linear response elimination of small component formalism, LRESC, together with some applications to few selected response properties. New physical insights do appear when the LRESC model is used to analyze the effect of the environment on magnetic shieldings, and when one search for the relativistic extension of well‐known nonrelativistic relationships like Flygare's relation among the NMR magnetic shielding and the nuclear spin‐rotation constant. A similar relationship is found for the g‐tensor and the susceptibility tensor.