Theoretica chimica acta

, Volume 84, Issue 1, pp 95-103

First online:

Internally contracted multiconfiguration-reference configuration interaction calculations for excited states

  • Peter J. KnowlesAffiliated withSchool of Chemistry and Molecular Sciences, University of Sussex
  • , Hans-Joachim WernerAffiliated withFakultät für Chemie, Universität Bielefeld

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The calculation of electronically excited states with the internally contracted multiconfiguration-reference configuration interaction (CMRCI) method is discussed. A straightforward method, in which contracted functions for all states are included in the basis, is shown to be very accurate and stable even in cases of narrow avoided crossings. However, the expense strongly increases with the number of states. A new method is proposed, which employs different contracted basis sets for each state, and in which eigensolutions of the Hamiltonian are found using an approximate projection operator technique. The computational effort for this method scales only linearly with the number of states. The two methods are compared for various applications.

Key words

Quantum chemistry Configuration interaction Excited states