Semiempirical methods flexibility

A more balanced description thus requires multiconfiguration self-consistent field (MCSCF)-based methods, where the orbitals are optimized for each particular state or optimized for a suitable average of the desired states (state-averaged MCSCF). In semiempirical methods, however, an MCSCF procedure is normally not required due to the limited flexibility of the minimal valence atomic orbital basis commonly used in these methods. Instead, a multireference Cl method including a limited number of suitably chosen configurations will be appropriate. 

Hyperfine coupling constants (hfcc) are a property on which much computational (as well as experimental) effort has been spent. Early success with semiempirical methods, such as INDO (16), in reproducing hfccs from ESR was accompanied by inconsistency of results. It was discovered that fortuitous cancellation of errors was behind the apparent agreement (17). As technical capabilities improved, it became clear that accurate computation of hfccs is a subtle problem, requiring both a basis set with flexibility near the magnetic nuclei and a substantial amount of electron correlation. At the present time, calculation of hfccs is essentially a resolved question The effort needed in ab initio calculations to achieve accurate results is known (18), and recent work with DFT theory seems to indicate that hfccs calculated by DFT methods are in quite satisfactory agreement with experiment (19). Even molecules that have been considered "pathological" can be correctly described with a systematic theoretical approach (20). 

Nevertheless, the combination of generality and flexibility makes ab initio methods a powerful complement to experimental measurements as well as to other computational techniques for small to intermediate size systems. Ab initio methods can, in principle, be applied to any geometry on the ground state or any excited state potential energy surface. Furthermore, with modern computer programs the quality of the wavefunction can be easily improved, in contrast to molecular force field or semiempirical methods that are comparatively difficult for the user to improve systematically. 

Stewart JJ (2007) Optimization of parameters for semiempirical methods V modification of NDDO approximations and application to 70 elements. J Mol Model 13 1173-1213 Van der Spoel D, Lindahl E, Hess B, Groenhof G, Mark AE, Berendsen HJ (2005) GRO-MACS fast, flexible, and fi-ee. J Comput Chem 26 1701-1718... 

One of these semiempirical methods scales the volume contributions to Tg of the diffeent moieties existing in the chemical structure of the repeating unit of polymers [31, 32]. Schneider and DiMarzio [33] suggested the more simple correlation of Tg with the mass/ flexible bond of the repeating unit. 

The principal developer of the code has been Frank Neese, first at the Max Planck Institute for Bioinorganic Chemistry in Miilheim and then at the University of Bonn. In the words of the program s author, ORCA is a flexible, efficient and easy-to-use general purpose tool for quantum chemistry with specific emphasis on spectroscopic properties of open-shell molecules. It features a wide variety of standard quantum chemical methods ranging from semiempirical methods to DFT to single- and multireference correlated ab initio methods. It can also treat environmental and relativistic effects. An abbreviated list of program s features includes ... 

A second type of restriction concerns the objective of the method. It is axiomatic that no single semiempirical method can accurately predict all properties the limitations in flexibility due to the use of a restricted basis set preclude that. Therefore, the choice of reference data should be limited to those data that help define the properties that the method is... 

At the time of this writing, it must be conceded that there have been no fundamental principles-based mathematical model for Nafion that has predicted significantly new phenomena or caused property improvements in a significant way. Models that capture the essence of percolation behavior ignore chemical identity. The more ab initio methods that do embrace chemical structure are limited by the number of molecular fragments that the computer can accommodate. Other models are semiempirical in nature, which limits their predictive flexibility. Nonetheless, the diversity of these interesting approaches offers structural perspectives that can serve as guides toward further experimental inquiry. 

Bis(crown ethers) connected by a flexible spacer are a source of intramolecular sandwich-type complexes with alkali metal ions <1992MI1>. A conformational analysis (based on a combination of semiempirical and ab initio methods) performed on 12-crown-3 38 and 12-crown-4 39 predicted that, in the case of sandwich-type complexation, the nucleophilic cavity of 12-crown-3 rather than that of 12-crown-4 is more prone to complexation with the Na+ ion. Accordingly, ion-selective electrodes based on bis(12-crown-3) derivatives with dialkylmalonate spacers displayed the highest selectivity for Na+ ions among the alkali and alkaline earths investigated, and superior to the Na+ selectivity reached with the bis( 12-crown-4) analogue <2003ANA(480)291>. 

Such specialized semiempirical treatments exist (see Ref. [42] for a review on early work), many of which are based on the MNDO model in one of its standard implementations. For example, there are several early MNDO and AMI variants with a special treatment of hydrogen bonds [42], which have recently been supplemented with an elaborate PM3-based parametrization for water-water interactions named PM3-PIF (pair interaction function) [69]. These special approaches exploit the flexibility offered in MNDO-type methods by the presence of the effective core-core repulsion term which can be modified for fine tuning. 

Methods for searching the conformational space of small and medium-sized molecules have been reviewed [969]. A general problem in the prediction of 3D structures of flexible drugs is the fact that conformations in vacuo i.e. all conformations calculated by force field, semiempirical, or ab initio methods) may be significantly different from the conformations in aqueous solution (2D NMR... 

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