Electron-correlated calculations, nuclear

Infrared, Raman, microwave, and double resonance techniques turn out to offer nicely complementary tools, which usually can and have to be complemented by quantum chemical calculations. In both experiment and theory, progress over the last 10 years has been enormous. The relationship between theory and experiment is symbiotic, as the elementary systems represent benchmarks for rigorous quantum treatments of clear-cut observables. Even the simplest cases such as methanol dimer still present challenges, which can only be met by high-level electron correlation and nuclear motion approaches in many dimensions. On the experimental side, infrared spectroscopy is most powerful for the O—H stretching dynamics, whereas double resonance techniques offer selectivity and Raman scattering profits from other selection rules. A few challenges for accurate theoretical treatments in this field are listed in Table I. 

Lantto, P. and Vaara, J. (2006) Calculations of nuclear quadrupole coupling in noble gas-noble metal fluorides Interplay of relativistic and electron correlation effects. Journal of Chemical Physics, 125, 174315-1-174315-7. 

The first topic has an important role in the interpretation and calculation of atomic and molecular structures and properties. It is needless to stress the importance of electronic correlation effects, a central topic of research in quantum chemistry. The relativistic formulations are of great importance not only from a formal viewpoint, but also for the increasing number of studies on atoms with high Z values in molecules and materials. Valence theory deserves special attention since it improves the electronic description of molecular systems and reactions with the point of view used by most laboratory chemists. Nuclear motion constitutes a broad research field of great importance to account for the internal molecular dynamics and spectroscopic properties. 

There has been interest in the theoretical treatment of the spectra of the phenanthrolines. Correlations between nuclear magnetic resonance (NMR)42-48 and electronic spectra22,38,40 41,49-51 with various quantum chemical data have been discussed. Often there is reasonable agreement between the calculated and measured spectra. 

The infrared spectra and dielectric constants,101,102 electronic,103-111 nuclear magnetic resonance (NMR),106,112-121 and mass122-126 spectral data for these compounds have been analyzed. There exist numerous quantum mechanical calculations of the electronic transitions and involvement of the sulfur electrons.107,111 Ultraviolet spectral determinations of the basicity of thiochroman-4-ones110 allowed correlations with the corresponding chroman-4-ones. 

There are two basic approaches to the theory of atomic helium, depending on whether the nuclear charge Z is small or large. For low-Z atoms and ions, the principal challenge is the accurate calculation of nonrelativistic electron correlation effects. Relativistic corrections can then be included by perturbation theory. For high-if ions, relativistic effects become of dominant importance and must be taken into account to all orders via the one-electron Dirac equation. Corrections due to the electron-electron interaction can then be included by perturbation theory. The cross-over point between the two regimes is approximately Z = 27... 

J. Gauss, Effects of electron correlation in the calculation of nuclear magnetic resonance chemical shifts, J. Chem. Phys. 99 (1993) 3629. 

With the success of these calculations for isolated molecules, we began a systematic series of supermolecule calculations. As discussed previously, these are ab initio molecular orbital calculations over a cluster of nuclear centers representing two or more molecules. Self-consistent field calculations include all the electrostatic, penetration, exchange, and induction portions of the intermolecular interaction energy, but do not treat the dispersion effects which can be treated by the post Hartree-Fock techniques for electron correlation [91]. The major problems of basis set superposition errors (BSSE) [82] are primarily associated with the calculation of the energy. 

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