Calibration electronic structures

Almost all contemporary ab initio molecular electronic structure calculations employ basis sets of Gaussian-type functions in a pragmatic approach in which no error bounds are determined but the accuracy of a calculation is assessed by comparison with quantities derived from experiment[l] [2]. In this quasi-empirical[3] approach each basis set is calibrated [4] for the treatment of a particular range of atoms, for a particular range of properties, and for a particular range of methods. Molecular basis sets are almost invariably constructed from atomic basis sets. In 1960, Nesbet[5] pointed out that molecular basis sets containing only basis sets necessary to reach to atomic Hartree-Fock limit, the isotropic basis set, cannot possibly account for polarization in molecular interactions. Two approaches to the problem of constructing molecular basis sets can be identified ... 

Although the most accurate calculations are usually not possible on systems of biological interest, a careful calibration of each level in the hierarchy ensures that even the results of the simpler calculations may be used with confidence. Indeed, the presence of such systematic, universally adopted hierarchies is probably the most distinctive feature of modern wave function-based quantum chemistry, setting it apart from other computational techniques of electronic structure. 

More recent calculations have shown qualitatively similar descriptions of plastocyanin, reproducing the Cu-S(thiolate) 7T bonding interaction in the HOMO, however, the sulfur 3p character in the HOMO shows a large dependence on the functional used. Here it should be emphasized that spectroscopic results have provided a very detailed description of the electronic structure of blue copper sites, which have been important for the calibration of electronic-structure calculations. ... 

In this article we have shown that the electronic structures of transition metal compounds based on a cubic architecture can be rationalized, by use of results obtained from molecular orbital calculations, leading to some interesting extensions of the electron-counting PSEP rules. Indeed, the cluster topology of the different cubic cluster categories is highly dependent on several parameters which can be calibrated. 

The values (or functional forms) of the energy integrals may be used as a method of forcing a particular model of molecular electronic structure to agree with experiment. That is, we can calibrate a particular model against experiment for some chosen property. 

In a first step, the electronic structure of the polyaniline base form has been followed versus the oxidation state. After that, we have proceeded further with the protonated emeraldine form. In all cases, the binding energy scale has been calibrated by setting the Cls core level main peak at 284.4 eV. 

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