J. Chem. Phys., 125 , 104106 (2006)

Danylo Katsa, Tatiana Koronab, and Martin Schütza

a Institute of Physical and Theoretical Chemistry, University of Regensburg, Universitätsstraße 31, D-93040 Regensburg, Germany, b Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland

Local CC2 electronic excitation energies for large molecules with density-fitting


A new local method for the computation of electronic excitation energies of singlet states in extended molecular systems is presented. It is based on the CC2 model and local approximations to the wave functions. In the proposed method the singles excitations are treated non-locally and local restrictions are imposed on doubles amplitudes only. The accuracy of the new method was tested by calculating several lowest excited states for 14 molecules and comparing them with canonical CC2 values. Deviations of the local excitation energies from the canonical reference values do not exceed 0.05 eV for all test molecules and all states in the lower energy range investigated in this work. The method uses the Density-Fitting approximation for all two-electron integrals, what considerably simplifies the computational complexity of the individual diagrams. A combination of the local approximations and the powerful Density-Fitting technique leads to a low-scaling method, capable to treat molecular systems comprising hundred atoms and more in a basis of a polarized double zeta quality. A test calculation for a system consisting of 127 atoms and 370 active electrons without symmetry is presented to show the efficiency of the new method.

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Tatiana Korona 13.06.2006