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Excited States of Large Open-Shell Molecules: An Efficient, General, and Spin-Adapted Approach Based on a Restricted Open-Shell Ground State Wave function
journal contribution
posted on 2016-02-19, 12:03 authored by Michael Roemelt, Frank NeeseA spin-adapted
configuration interaction with singles method that
is based on a restricted open-shell reference function (ROCIS) with
general total spin S is presented. All excited configuration state
functions (CSFs) are generated with the aid of a spin-free second
quantization formalism that only leads to CSFs within the first order
interacting space. By virtue of the CSF construction, the formalism
involves higher than singly excited determinants but not higher than
singly excited configurations. Matrix elements between CSFs are evaluated
on the basis of commutator relationships using a symbolic algebra
program. The final equations were, however, hand-coded in order to
maximize performance. The method can be applied to fairly large systems
with more than 100 atoms in reasonable wall-clock times and also parallelizes
well. Test calculations demonstrate that the approach is far superior
to UHF-based configuration interaction with single excitations but
necessarily falls somewhat short of quantitative accuracy due to the
lack of dynamic correlation contributions. In order to implicitly
account for dynamic correlation in a crude way, the program optionally
allows for the use of Kohn–Sham orbitals in combination with
a modest downscaling of two-electron integrals (DFT/ROCIS). All two-electron
integrals of Kohn–Sham orbitals that appear in the Hamiltonian
matrix are reduced by a total of three scaling parameters that are
suitable for a wide range of molecules. Test calculations on open-shell
organic radicals as well as transition metal complexes demonstrate
the wide applicability of the method and its ability to calculate
the electronic spectra of large molecular systems.