Theoretical spectroscopic constants

Many of these points are well illustrated by Cu2, which has become a benchmark for theoretical calculations owing to its relative simplicity and the availability of accurate experimental data. The theoretical spectroscopic constants are quite poor unless the 3d electrons are correlated, even though both Cu atoms nominally have a 3d °4s occupation. In fact, quantitative agreement with experiment is achieved only if both the 3d and 4s electrons are correlated, both higher excitations and relativistic effects are included, and large one-particle basis sets, including several sets of polarization functions, are used (24,25). This level of treatment is difficult to apply even to Cua, let alone larger Cu clusters. 

P, Hennig, W. P, Kraemer and 6. H. F. Oierckseni A Compilation of Theoretical Spectroscopic Constants and Rotational-Vibrational Transition Frequencies for the Isoelectronic Series of Linear Triatomic Molecules HCN, HNC, HCO , HOC", HNN Obtained from Ab Initio Calculated Energy Hypersurface, Max-Plenck-Insti-tut fur Physik und Astrophysik, Munich 1977. 

The study of molecular systems containing metal atoms, particularly transition metal atoms, is more challenging than first-row chemistry from both an experimental and theoretical point of view. Therefore, we have systematically studied (3-5) the computational requirements for obtaining accurate spectroscopic constants for diatomic and triatomic systems containing the first- and second-row transition metals. Our goal has been to understand the diversity of mechanisms by which transition metals bond and to aid in the interpretation of experimental observations. 

The answer to the question why calculate spectroscopic constants is not merely to find the value . There are in fact two possible reasons why one should use theoretical methods to compute spectroscopic constants. Firstly, there are cases where an experimental value has been measured for some constant, but the observed magnitude or even sign is not comprehensible in terms of the idea of electronic structure which the spectroscopist has in mind. In such cases a relatively crude calculation which only reproduces the observation to an order of magnitude may offer explanations in terms of perturbations by unobserved states or the atomic constitution of molecular orbitals. 

Table 6-6. Spectroscopic constants of the nine low-lying states of Tl2 at the DC-CASPT2 level with previous theoretical results...

Ground State of CO. The CO molecule has been extensively studied, both experimentally and theoretically. Table 14 compares ground-state 02+) spectroscopic constants calculated by the Hartree-Fock method109 and by the density functional approach99 with experiment.104 In addition to these spectroscopic constants, the polar nature of the molecule provides a further measurable quantity, the dipole moment. Since the intensities of infrared vibration-rotation bands allow the dipole moment to be determined as a function of C-O separation this provides a useful comparison with the results of ab initio calculations. For example, the positive sign obtained from the equilibrium dipole moment by Hartree-Fock calculations was viewed as a reason to question the negative value found experimentally, whereas the current view is that the positive sign is a defect of the Hartree-Fock method. 

Although force constants belong to the so called "spectroscopic constants", they are not genuine observables but only values inferred from the theoretical treatment of the assumed force field. It is usual to express the force field in the form of a power series... 

All spectroscopic constants were obtained from Gordon (3 ). Theoretical calculations by Kruba ( ) predict a ground state. The recent review by Huber and Herzberg (5) did not indicate any experimental data available on the vibrational-rotational constants for BC(g). 

Table 5.6 Comparison of Eg from Dielectric Constant with Theoretical Spectroscopic Gap...

In the previous sections, we have discussed the application of the most accurate theoretical methods for the calculation of spectroscopic constants. Another important area of computational chemistry is dynamics. A variety of... 

For the theoretical determination of spectroscopic constants of weakly bound van der Waals molecules like Ru2, a highly accurate treatment for both electron correlation and relativity becomes necessary to obtain reliable results [123]. Size-extensive correlated relativistic calculations are desirable, but at present these straightforward approaches are computationally too demanding because of the large basis sets required including multiple polarization functions, suggesting that the RECP approximation is the method of choice. 

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