Concept of effective states of atoms in compounds to describe properties determined by the densities of valence electrons in atomic cores

We propose an approach for describing the effective electronic states of atoms in compounds to study the properties of molecules and condensed matter, which are circumscribed by the operators heavily concentrated in atomic cores. Among the properties are hyperfine structure, space parity (P) and time reversal invariance (T) nonconservation effects, chemical shifts of x-ray emission lines (XES), Mossbauer effect, etc. An advantage of the approach is that a good quantitative agreement of predicted and experimental data can be attained even for such difficult cases as XES chemical shifts providing correct quantum-mechanical interpretation of the experimental data. From the computational point of view the method can be quite efficient being implemented in the framework of the relativistic pseudopotential theory [A. V. Titov and N. S. Mosyagin, Int. J. Quantum Chem. 71, 359 (1999)] and procedures of recovering the wave functions in heavy-atom cores [A. V. Titov, N. S. Mosyagin, A. N. Petrov, and T. A. Isaev, ibid. 104, 223 (2005)] after a molecular, cluster or periodic structure calculation performed on the basis of pseudo-orbitals smoothed near the nuclei within the pseudopotential approximation. We report results of our studies of a number of atomic and molecular systems to demonstrate the capabilities of the approach.