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RESEARCH PRODUCT

Ab Initio Methods for Excited States

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This chapter focuses mainly on the performance of ab initio methods for the description of spectroscopic molecular properties of compounds. Most of the quantum-chemical methods developed up to date are based on the concept of the one-electron wave function. The electronic states of a system with N electrons are described by a double expansion. Molecular orbitals (MOs) are one-electron wave functions expressed as linear combinations of a known one-electron basis set (K) and the N electron wave function is formulated in a many-electron basis set formed by determinants (or linear combination of them to form spin-adapted wave functions), built as normalized antisymmetric products of MOs. According to the number of configurations considered, the methods are classified in the following two categories: (1) Single-configuration methods—they are typically based in the Hartree-Fock (HF) reference, which determines the MOs. The electron correlation treatment is usually performed at the configuration interaction (CI), coupled-cluster (CC) or Moller-Plesset (MP) levels. (2) Multiconfigurational methods—part of the electronic correlation is already included in the reference wave function, normally by using a MCSCF wave function that determines a set of MOs. The remaining electron correlation effects are accounted for by muti-reference CI (MRCI), multireference coupled-cluster (MRCC) or multireference perturbation theory (MRPT) techniques.