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The higher CN values dramatically increase the number of possible compounds that can be generated around a metal center. Flexible bonding makes it hard to simulate diverse chemical bonds with one modeling strategy. Moreover, the number of electronic states available to some metal ions coupled with more flexible bonds results in many configurations having very similar energies that in turn require highly accurate and expensive computational methods to differentiate the subtle differences. In typical force fields for metal ions, parameters are designed for a specific CN found for the metal ion in an environment.
Because an individual metal ion can form many complexes with different CNs, which may or may not be close in energy, and may change from one environment to another, it is hard to simulate these systems accurately. Hence, generating a general force field for metal ions becomes more of a challenge. On the basis of the previous discussion, we hope we have lain bare why it is such a challenge to model metal-containing compounds. Unfortunately, there are limited experimental data about TM species, which slows the development of accurate theoretical methods to model TM-containing compounds.
For example, both the number and the accuracy of experimentally derived heats of formation HOFs for TM-containing molecules are modest when compared to what is available for organic compounds. In the work of Jiang et al. As compared to the number of possible TM-containing molecules that could exist, this number represents a rather paltry validation set. These results indicate that to impel theoretical methods forward, extensive and highly reliable experimental data will be essential.
Because it is challenging to obtain thermodynamic experimental data for TM species to chemical accuracy, the ability for computational methods to advance will be hindered as well. The value in brackets represents the number of data points in the subset. Copyright American Chemical Society. Another challenging issue is the way in which accurate hydration free energies HFEs are obtained for ions.
Chemists determine the HFE of a single ion based on experimentally determined HFE values of salts coupled with theoretical assumptions. Meanwhile, there are also other issues that should be considered, including the difference between intrinsic and absolute HFE values, if one wants to accurately model HFE values. Related discussions can be found in the work of Grossfield et al.
Another aspect that restricts the development of TM modeling is inconsistencies between the available experimental and computational data. For example, X-ray diffraction XRD data are always based on the crystal phase. In QM or molecular mechanics MM modeling efforts, the calculation is usually performed in the gas phase without inclusion of crystal packing effects.
Meanwhile, using experimental data directly may introduce inaccuracies due to different interpretations of the data.
For example, the van der Waals VDW radii used in X-ray studies are based on the distance of nearby atoms, which are influenced by the crystal packing, while the theoretical chemist treats the VDW radii sum as the energy minimum of two isolated particles. Moreover, experimental thermochemical data are more often given as free energy values. However, theoretical computation of free energy changes requires extensive sampling, which is still challenging, especially for macromolecular systems. This Review covers the theoretical and computational work on metal ion modeling reported in the past several decades, with a focus on metal ion modeling employing classical force fields.
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Because quantum calculations are usually used for parametrization and to benchmark metal ion force fields, we briefly review extant QM modeling strategies with respect to metal-containing systems in section 2. Next, we review the widely used nonbonded and bonded approaches to classical modeling of metal ions in sections 3 and 4 , respectively.
We then summarize the cationic dummy atom and combined models in section 5. Subsequently, we review several widely used polarizable models in section 6 , while in section 7 we review classical models based on the angular overlap model AOM and valence bond VB theory. Finally, we summarize our conclusions and perspectives on the prospects of this research field in section 8. While we strive to give a balanced review of the field, we realize that due to the limits of our knowledge omissions might occur, for which we make our apologies. Note that the dates listed in the main text represent the year of publication in the original article, which may differ from the year when the article appeared in press.
For example, the original paper describing the LJ-type nonbonded model for divalent metal ions was published online in October of , while it appeared in print in in the Journal of Chemical Theory and Computation.
[Solution manual] classical mechanics, goldstein
Hence, we refer to the work as being reported in A broad range of reviews regarding the computational modeling of metal ions have been published in the past several decades. Below, we summarize some extant reviews for the sake of the readers who want to trace back through the relevant literature. There are a number of reviews concerning TM chemistry that have been published in Chemical Reviews. Holm and Solomon coedited a series of reviews largely focused on experimental studies of bioinorganic enzymes.
In total, 13 articles on various topics of QM TM modeling were included. Noodleman et al.
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Finally, Odoh et al. Besides the aforementioned collections of relevant review articles, several other germane reviews have been published in the last several decades. Rode et al. Deeth reviewed the application of DFT methods to the modeling of the active sites and mechanisms of metalloenzyme systems in In , Shaik et al.
There are a few reviews on classical modeling of inorganic complexes published several decades ago.
For example, Brubaker and Johnson reviewed MM modeling in coordination chemistry in Norrby and Brandt reviewed parametrization strategies for deriving MM parameters for coordination complexes in Banci reviewed MD simulation work on metalloproteins, and Comba and Remenyi reviewed MM-based modeling of inorganic compounds both in Because there are very few extant review articles specific to the modeling of metal ions using polarizable force fields, we decided to highlight a number of reviews on polarizable force fields that have appeared regardless of their overall relevance to metal ion-containing systems.
In this way, interested readers can gain entry into this rapidly developing field. In the collection of Chemical Reviews articles edited by Holm and Solomon in , 52 one article by Stephens et al. Loopes et al. A number of more extensive reviews on QM modeling of TM-containing systems are available in the literature for the interested reader.
In particular, modeling the multireference characteristic of many TM species using single reference methods is not handled correctly because they do not address static correlation. Besides modeling molecules, it is hard to simulate the metal atoms from the middle TM series. Here, we treat the Ni atom as an example.
The atomic excitation energies of Ni for 3d 9 4s 1 —3d 8 s 2 3 D— 3 F and 3d 9 4s 1 —3d 10 3 D— 1 S excitations calculated by several different QM approaches are shown in Table 1 , together with experimental values.
Table 1 shows that the HF method gives significant errors predicting these properties, while multireference methods need to be employed to obtain accurate results. Martin and Hay have studied the excitation energies of low-lying states and ionization potentials IPs of TMs. Bauschlicher et al. Finally, they showed that the multireference configuration interaction MRCI method coupled with large atomic natural orbital ANO basis sets could further reduce the errors see Table 1.
Because the DFT method considers the electron density—energy relationship directly, it incorporates static correlation effectively when modeling TM containing systems.
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Hence, the answer to the question of which functional should be used really depends on the system and property one wants to investigate. In general, correlation functionals, which do not attain the homogeneous gas electron limit, are not suggested for pure metal systems, , while for mixed systems, such as organometallics, this is not a significant limitation.
However, because it neglects the correlation energy and is a single reference method, it is generally not used for modeling TM-containing systems. Even with these issues, HF methods can be used for force constant derivation with some success. Multiple reference wave function-based methods offer better descriptions but with greatly increased computational cost. TM-containing systems are more likely to have strong multireference properties due to the near degeneracy of d orbitals. A well-known multireference TM system is Cr 2 , its multireference character making it challenging to simulate using methods employing a single Slater determinant.
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For example, the C 2 molecule, which has several low-lying excited electronic states, also has strong multireference character. Abrams and Sherrill have explored the limitations of single-reference methods in modeling the multireference C 2 system. Jiang et al.