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K. B. Be´ c et al.
Fig. 5.4 Interatomic potential (solid black) and the associated harmonic approximation (dashed
black) of the HCl molecule in the gas phase at CCSD(T)/aug-cc-pVQZ level and the respective vibrational wavefunctions (red) obtained via the Numerov approach. The associated energy differences
between the individual states correspond to the wavenumbers measured via vibrational spectroscopy
made into quantum vibrational spectroscopy, the electronic structure of the system
under consideration needs to be determined [6]. This can be accomplished with a
large array of different approaches, aimed at providing various approximations to
balance the accuracy of results and the computational demand.
5.3.2 Quantum Chemical Methods for the Determination
of the Electronic Structure of Molecular Systems
An approach for the determination of the electronic structure of a quantum system
(Fig. 5.5) may follow two principal ways of categorization; conceptual and practical. From the conceptual point of view, these methods differ by how the energy of
the system is described. The major approaches will be presented in Sects. 5.3.2.1,
5.3.2.2, 5.3.2.3, 5.3.2.4, 5.3.2.5, and 5.3.2.6. More exhaustive information can be
found in topic-oriented textbooks [6]. The practical categorization mostly concerns
their computational complexity, with the cost versus accuracy factor of the available
methods being a key consideration, which translates into their respective applicability
to certain problems.