Response to Geometric Distortions from Atom-Based Relativistic Projection on Electronic States

We suggest a quasi-relativistic model for molecular electronic structure calculations obtained by an atomic ansatz for the relativistic projection transformation. With such a choice, the projection transformation becomes both transferable and independent of the geometry. The formulation is flexible with regard to the choice of the projection transformation. We employ the free-particle Foldy-Wouthuysen and the Douglas-Kroll second-order variants for the projection coefficients. First results for the diatomic molecules AuH, AuCl, and Au2 and two structural isomers of Ir4 suggest the new approximate method as an efficient and accurate replacement for the conventional transformation