Ab initio energetics

Abstract. The physical nature of nonadditivity in many-particle systems and the methods of calculations of many-body forces are discussed. The special attention is devoted to the electron correlation contributions to many-body forces and their role in the Be r and Li r cluster formation. The procedure is described for founding a model potential for metal clusters with parameters fitted to ab initio energetic surfaces. The proposed potential comprises two-body, three-body, and four body interation energies each one consisting of exchange and dispersion terms. Such kind of ab initio model potentials can be used in the molecular dynamics simulation studies and in the cinalysis of binding in small metal clusters. 

N. Govind et al Accurate ab initio energetics of extended systems via explicit correlation embedded in a density functional environment. Chem. Phys. Lett. 295, 129 (1998)... 

Botschwina later used a doubly polarized basis set to study this complex, along with a CEPA-1 treatment of electron correlation. The ab initio energetics were fit to an analytic four-dimensional function in order to elucidate anharmonic effects. The results at various levels of theory are presented in Table 3.64 along with experimentally measured quantities. Comparison of the SCF and CEPA-1 data suggests that while correlation yields major changes in the frequencies themselves, the shifts that occur upon complexation are surprisingly insensitive to correlation. The same is true of introduction of anharmonicity with one major exception. Whereas the frequency shifts of the stretches of the HCN proton acceptor molecule are little affected by introduction of anharmonicity, the red shift of HP is increased by 46% from 168 to 245 cm . This latter result is in near perfect agreement with... 

The idea of partitioning is that in the course of an ab initio calculation, different elements can be extracted and/or isolated. At the most basic level, this serves as a means of interpreting ab initio energetics more so than a distinct means for obtaining the energetics. Morokuma [123 125] and Kollman [126] devised the key computational strategies to extract from ab initio calculations the contributions that could be associated with the different elements of noncovalent weak interaction. One immediate outcome was the confirmation that electrical... 

Gresh, N., Policar, C., and Giessner-Prettre, C. (2002). Modeling copper(i) complexes SIBFA molecular mechanics versus ab initio energetics and geometrical arrangements,/ Phys. Chem. A 106,23, pp. 5660-5670. 

The second generation force fields for nucleic acids were designed to be used with an explicit solvent representation along with inclusion of the appropriate ions [28,29]. In addition, efforts were made to improve the representation of the conformational energetics of selected model compounds. Eor example, the availability of high level ab initio calculations on the conformational energetics of the model compound dimethylphosphate yielded... 

Most of the present implementations of the CPA on the ab-initio level, both for bulk and surface cases, assume a lattice occupied by atoms with equal radii of Wigner-Seitz (or muffin-tin) spheres. The effect of charge transfer which can seriously influence the alloy energetics is often neglected. Several methods were proposed to account for charge transfer effects in bulk alloys, e.g., the so-called correlated CPA , or the screened-impurity model . The application of these methods to alloy surfaces seems to be rather complicated. 

We have shown that a tight-binding scheme accounts for many physical phenomena which are of importance for the understanding of the energetics of surface defects and of crystal growth. This method which is simple and much less computer time demanding than ab initio methods is thus very useful to derive trends along the transition series. 

Supplementary to this subject, a recently published ab-initio calculation of silicondicarbonyl 0 = C = Si = C = 0 should be mentioned [195]. With an extended basis set (DZP functions) a bent C2 structure with C—Si—C 80.0°, Si—C 1.871, C —O 1.126 A is calculated the bent conformation is energetically more stable by 76.7 kcal/mol than a linear Dxh structure [196], These results also allow a reinterpretation of the experimental data (IR spectroscopy) for silicondicarbonyl [197]. 

We have considered the larger AI4-AI6 clusters using both ab initio calculations and the parameterized model (9). The results for AI4 and AI5, summarized in Table IV, show that the parameterized model and ab initio calculations agree well on the relative energetics if both the two- and three-body interactions are included. For Ale it is difficult to treat all the structures at the TZ2P-CPF level, but for the structures considered, there is reasonable agreement between the ab initio and model results. 

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