Zoila Barandiarán and Luis Seijo work at the Department of Chemistry of Universidad Autónoma de Madrid, Madrid, Spain. They accumulated a long experience on development and application of quantum chemical ab initio methods to local states of impurities in ionic crystals.
In 1988 they developed the ab initio model potential (AIMP) embedding method [1] as a way to include Coulomb, exchange, and Pauli repulsion embedding effects from a solid host onto an active cluster. Few years later they implemented it in the program MOLCAS, which is especially indicated to handle excited states of molecules with multi-referential multi-configurational wave functions; this enabled the performance of highly sophisticated wave function calculations on local excited states of solids including embedding effects and non-dynamic and dynamic correlation [2]. After some years of applications mostly to first-series transition metal ions in solid hosts, their interest on heavier elements, in which scalar and spin-orbit coupling relativistic approaches are compulsory, led them to develop an ab initio spin-orbit coupling relativistic approach [3]. After using it for about a a decade, they moved to the theoretically more sound Douglas-Kroll-Hess method (DKH) [4], which they use systematically now. In order to handle together dynamic correlation effects in multiconfigurational wave function methods and spin-orbit coupling effects, they developed an approximated decoupling method which is applicable to wave function methods based on configuration interaction, the spin-free state shifting method [5]. This method was implemented in MOLCAS for Restricted Active Space wave functions under the name RASSI-SO [6] and they are now using it systematically.
In 2000 they started to work with excited states of f-elements, both lanthanides and actinides, and few years later most of their work was focused on these elements, with emphasis on the luminescent properties of the active centers they create when doped in ionic hosts. They recently reviewed their work on this area [8] and published a perspective review on ab initio calculations of excited states of lanthanides in solids [9].
[1] Z. Barandiarán and L. Seijo. The Ab Initio Model Potential representation of the crystalline environment. Theoretical study of the local distortion on NaCl:Cu(+). J. Chem. Phys., 89, 5739 (1988). [http://dx.doi.org/10.1063/1.455549]
[2] Karlstrm G, Lindh R, Malmqvist PA, Roos BO, Ryde U, Veryazov V, et al. MOLCAS: a program package for computational chemistry. Comput. Mater. Sci. 28, 222 (2003). [http://dx.doi.org/10.1016/S0927-0256%2803%2900109-5]
[3] L. Seijo. Relativistic ab initio model potential calculations including spin-orbit effects through the Wood-Boring Hamiltonian. J. Chem. Phys., 102, 8078 (1995). [http://dx.doi.org/10.1063/1.469007]
[4] M. Douglas and N. M. Kroll. Quantum electrodynamical corrections to the fine structure of Helium. Ann. Phys. (N.Y.) 82, 89 (1974). B. A. Hess. Relativistic electronic-structure calculations employing a two-component no-pair formalism with external-field projection operators. Phys. Rev. A 33, 3742 (1986). T. Nakajima and K. Hirao, The Douglas-Kroll-Hess approach. Chem. Rev. 112, 385 (2012). [http://dx.doi.org/10.1021/cr200040s]
[5] R. Llusar, M. Casarrubios, Z. Barandiarán, and L. Seijo. Ab initio model potential calculations on the electronic spectrum of Ni2+-doped MgO including correlation, spin-orbit and embedding effects. J. Chem. Phys., 105, 5321 (1996). [http://dx.doi.org/10.1063/1.472376]
[6] P.-A. Malmqvist, B. O. Roos, and B. Schimmelpfennig. The RASSI aproach with spin-orbit coupling. Chem. Phys. Lett. 357, 230 (2002).
[7] L. Seijo and Z. Barandiarán, Structure and spectroscopy of Pa4+ defects in Cs2ZrCl6. An ab initio theoretical study. J. Chem. Phys., 115, 5554 (2001). [http://dx.doi.org/10.1063/1.1398092]
[8] Barandiarán Z, Seijo L. 4f, 5d, 6s, and impurity-trapped exciton states of lanthanides in solids. In: Dolg M, editor. Computational Methods in Lanthanide and Actinide Chemistry. New York: John Wiley & Sons; 2015. p. 217–240.
[9] Seijo L, Barandiarán Z. In: Bünzli JC, Pecharsky V, editors. Ab Initio Calculations on Excited States of Lanthanide Containing Materials. vol. 50 of Handbook on the Physics and Chemistry of Rare Earths. Elsevier; 2016. p. in press.