Supermolecule approach molecules

On one hand, there are the dielectric properties, which are especially important for polai solvents like water. Bulk properties can, on the other hand, only be modeled by using a supermolecule approach with explicitly defined solvent molecules. 

The simplest discrete approach is the solvaton method 65) which calculates above all the electrostatic interaction between the molecule and the solvent. The solvent is represented by a Active molecule built up from so-called solvatones. The most sophisticated discrete model is the supermolecule approach 661 in which the solvent molecules are included in the quantum chemical calculation as individual molecules. Here, information about the structure of the solvent cage and about the specific interactions between solvent and solute can be obtained. But this approach is connected with a great effort, because a lot of optimizations of geometry with ab initio calculations should be completed 67). A very simple supermolecule (CH3+ + 2 solvent molecules) was calculated with a semiempirical method in Ref.15). 

The alternative theoretical scheme for studying chemical reactivity in solution, the supermolecule approach, allows for the investigation of the solvation phenomena at a microscopic level. However, it does not enable the characterization of long-range bulk solvent forces moreover, the number of solvent molecules required to properly represent bulk solvation for a given solute can be so large that to perform a quantum chemical calculation in such a system becomes prohibitively expensive. ... 

Specific solute-solvent interactions, such as hydrogen bonding or protonation, may be included in the calculation of the shielding of solute nuclei by a supermolecule approach. The appropriate structure of the solute-solvent supermolecule may be obtained by the use of molecular mechanics simulations. At the semi-empirical MO level this approach has been successfully used to describe the effects of hydrogen bonding on the nuclear shielding of small molecules. 

Space does not permit the inclusion of any of the papers dealing with hydrogen bonding188 or solvation phenomena,189 and the reader is referred to the above references for more details. It is clear, however, from recent work that it is feasible to include the interaction with several solvent molecules using the supermolecule approach, and very interesting and informative results have been obtained in this way. 

In the second family of approaches, explicit solvent molecules are placed around the gas phase stationary point structures. In some cases, the gas phase geometries are held constant and only the geometries and/or positions of the surrounding solvent molecules are optimized, and in other cases, the structure of the whole system (often called a supermolecule 32) is optimized. The supermolecule approach generally only involves explicit solvent molecules from the first (and occasionally second) solvation shell of the solute. 

It is also possible to combine the supermolecule and continuum approaches by using specific solvent molecules to capture the short-range effects (i.e., those involving specific noncovalent interactions between solute and solvent) and a reaction field to treat longer range effects.33-35 Alternatively, structures along the gas phase reaction coordinate can be immersed in a box of hundreds (or more) of explicit solvent molecules that are treated using force field approaches.36,37 Each type of method - the SCRF, solvent box, and supermolecule approaches - tests the importance of particular features of the solvent on the reactivity of the solute dielectric constant, multiple specific classical electrostatic interactions, and specific local directional noncovalent interactions, respectively. 

Supermolecule model. By a "supermolecule" we imply a model consisting of the solute molecule surrounded by a certain number of solvent molecules. Pair complexes solute-solvent and solvent-solvent may be considered the simplest supermolecules. Since the cost of the supermolecule approach becomes prohibitive as the number of solvent molecules is increased, in most treatments only the first solvation shell is assumed. Such small clusters cannot of course provide a realistic model of a liquid but rather they give us a theoretical picture of what is referred as to "the solvation in the gas phase". As with the approach dealt with in the last paragraph, the ab initio calculation on the supermolecule should be followed by a statistical thermodynamic treatment. The use of the standard statistical thermodynamic is straightforward, in which case the supermolecule approach becomes e-quivalent to treatment of common chemical equilibria dealt with In Section 5.F. The calculations presented in Table 5.17 are just of this... 

In most cases, the solute consists of well-identified, neutral molecules that interact only weakly with each other. This means that reducing the solvent to consisting of just some few molecules around the solute may provide a reasonable approximation. In this case, the weak bonds i.e., hydrogen bonds or van der Waals bonds) to more distant solvent molecules are broken, but it is expected that this will lead to only insignificant electronic redistributions. Ultimately, the finite system consisting of the solute and the smaller number of solvent molecules can be treated with the accurate methods of the preceding subsection. This is the supermolecule approach. 

In Table 1 we summarize their main findings. For the frozen-density calculations they considered two different approaches, one where the solvent-molecule density was kept fixed and one where it was allowed to relax. In the table we have only shown the results for the latter, which according to the authors led to an improved accuracy. The table shows that the dipole and the quadrupole moments are very similar for both approaches, which is to a lesser extent the case for the excitation energies and the static (hyper)polarizabilities. The latter were calculated using time-dependent density-functional theory. In order to understand this discrepancy the authors used also a supermolecule approach with just two solvent molecules. By comparing with results from calculations with the frozen-density and the polarizable-molecule approaches on the same system they concluded that the frozen-density approach was the more accurate one in calculating the responses to electromagnetic fields. 

With a supermolecule approach, Bock et al. studied the first and second shell of water molecules surrounding various metal ions. They studied M (H20)i8 clusters using parameter-free density-functional calculations and trying to optimize the structures in largely unbiased calculations. Due to the size of the system (55 atoms, or 19 building blocks when assuming that the H2O units stay intact), this is a far from trivial endeavour (see, e.g., ref 2). 

TD-DFT) they calculated the transition energies and dipole moments for NMA both in vacuum and in an aqueous solution. Moreover, in the treatment of the solvent they compared two different approaches, i.e., a polarizable-continuum method (COSMO) and a supermolecule approach. For the latter, the authors performed molecular-dynamics calculations using a force-field model and, subsequently, extracted a cluster containing the solute and 3 water molecules that form hydrogen bonds to the solute. Averages over 90 such configurations were ultimately determined. 

Step 1. Find a monohydration sphere or a monohydration surface (limited only to the planes of the tautomers in the case of planar 7r-electron systems) for both the tautomers with a particular calculation method. This requires calculations (e.g., within the so-called supermolecule approach or with the use of electrostatic potentials) for the complex tautomer plus water in different conformations. In other words, one must move the water molecule either in the space around the tautomer... 

Among the few determinations of of molecular crystals, the CPHF/ INDO smdy of Yamada et al. [25] is unique because, on the one hand, it concerns an open-shell molecule, the p-nitrophenyl-nitronyl-nitroxide radical (p-NPNN) and, on the other hand, it combines in a hybrid way the oriented gas model and the supermolecule approach. Another smdy is due to Luo et al. [26], who calculated the third-order nonlinear susceptibility of amorphous thinmultilayered films of fullerenes by combining the self-consistent reaction field (SCRF) theory with cavity field factors. The amorphous namre of the system justifies the choice of the SCRF method, the removal of the sums in Eq. (3), and the use of the average second hyperpolarizability. They emphasized the differences between the Lorentz Lorenz local field factors and the more general Onsager Bbttcher ones. For Ceo the results differ by 25% but are in similar... 

In conclusion, it is clear that considerable information about a molecule s inherit stability, and its ability to interact with other chemical species, can be deduced from the electrostatic potential and some other well defined properties that reflect the molecular charge distribution. It should be emphasized that this approach only requires the wavefunction of the isolated molecule to be calculated, and it is therefore considerably more economical than the conventional supermolecule approach for calculation of intermolecular interaction energies. In particular, we believe that this methodology can be very useful for studying interactions in biological systems, since these often involve large molecules with several interaction sites. 

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