Local correlation method for metals: a step towards a general approach
Final Report Abstract
Highly accurate methods such as coupled cluster techniques can be used for periodic systems within the framework of the method of increments. In this approach, a periodic HF calculation is followed by a many-body expansion of the correlation energy, where the individual units of the expansion are either atoms or other domains of localised orbitals. Calculations based upon the method of increments have been performed on a variety of solids with band gaps. We have shown that the incremental scheme, after some reformulation, can also be applied when considering metals. By the start date of the project the method of increments has been successfully applied to several group 2 and 12 metals, namely: Mg, Zn, Cd, and Hg. An attempt to further extend this series yields some difficulties. For example, existence of the sp-d hybridisation of the valence band states of the fcc Ca and Sr in the vicinity of the Fermi level leads to the failure of the single-reference treatment in these cases. In order to deal with this issue, the multireference incremental scheme has been developed. Its application to the systems under study allows one to achieve very good agreement between computed and experimental cohesive energies, covering approximately 97 % of the experimental correlation energy. Still, all the studied cases are the closed shell metals. As a further step toward generalisation of this approach, the proper way for treatment the open shell systems (as e.g. bcc Na) has to be found. In this regard an embedding scheme relying on pairs of atoms has been suggested. With this embedding it is also possible to localise the orbitals in the central part of the considered finite fragment of the solid under study and to perform an incremental expansion for the correlation energy. Thus, the obtained results are shown to agree well with the experimental values for all considered bulk metals. An extension of the method of increments to a low-dimensional conducting system is also considered in the framework of this project. The test case here was the He/Mg(0001) system, which is particularly challenging among different He-metal surface systems due to the extremely small adsorption energy of He atoms. Experimental measurements have proven the preferential He atom adsorption on the on-top positions of metallic surfaces, instead of the expected hollow sites. The interpretation of such a behaviour was, however, a matter of debate. Our calculations show clear evidences of the anti-corrugation of the interaction potential. The analysis of the individual contributions to the correlation energy allows to conclude that such a behaviour is attributed to the screening of the He atom by the metal for the on-top configuration.
Publications
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“First multi-reference correlation treatment of bulk metals”, J. Chem. Theory Comput. 10, 1698 (2014)
E. Voloshina and B. Paulus
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“Post-Hartree-Fock studies of the He/Mg(0001) interaction: anti-corrugation, screening and pairwise additivity”, J. Chem. Phys. 144, 244707 (2016)
M. P. de Lara-Castells, R. Fernandez-Perea, F. Madzharova, and E. Voloshina
