  Scalar relativistic corrections to atomization energies of 1st-and 2nd-row
molecules can be rationalized in terms of a simple additive model, linear in
changes in atomic s populations. In a sample of 200 first-and second-row
molecules, such a model can account for over 98% of the variance (99% for the
first-row subset). The remaining error can be halved again by adding a term
involving the change in atomic p populations: those coefficients need not be
fitted but can be fixed from atomic electron affinity calculations. This model
allows a fairly accurate a priori estimate for the importance of scalar
relativistic corrections on a reaction energy, at essentially zero
computational cost. While this is not a substitute for explicit calculation of
Douglas-Kroll-Hess (DKH) or exact two-component (X2C) relativistic corrections,
the model offers an interpretative tool for the chemical analysis of scalar
relativistic contributions to reaction energies.
