Supermolecules geometry optimization

The basis set used for calculations were the STO-3G, the 3-21G, and the 6-31G basis sets, as implemented by the Gaussian-88 computer program. The energies of interaction were computed by using the supermolecule approach where the sum of the energies of the subsystems are substracted from the energy of the complex. All the species were geometry optimized. 

The calculated isotropic H and 14H hfcs on the geometry-optimized P />0 including effects from the surrounding are in remarkable good agreement with experimental values.202 The presented theoretical results show that P70o is also electronically a supermolecule consisting of two Chi a-type 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). 

Explicit calculation of the electronic coupling matrix element, Hah, is performed by modeling the transition state (Fig. 3) as a supermolecule, [M(H20)6]2+, and optimizing its geometry under the constraint of having an inversion center of symmetry The numerical value of Hab is then obtained from the energy gap between the appropriate molecular orbitals of the supermolecule. 

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. 

Figure 10. Superimposed optimized geometries from the gas-phase and COSMO calculations for (a) the reactant supermolecule of the hydration process rOl cis-Pt(NH3)2Cl,+H20 arrow points to the change of water position passing from the gas-phase (thin sticks) to die PCM structure (balls sticks), (b) the product supermolecule plO neutral (gas phase - thin sticks) and ion pair structures (PCM -ball sticks). Arrow shows the proton transfer from an aqua-ligand to a released NH/ panicle.

The many-body contributions are automatically taken into account in the supermolecule treatment, whatever the method. In the perturbation theory, the different terms are developed and computed independently. The analytical potentials used to optimize geometries or in Monte Carlo and Molecular Dynamic treatments are quite often deduced from approximations of the perturbation terms. During a long time, they were limited to the pair... 

Figure 15 Topologically chiral interlocked molecules that were studied by CD. (a) The structures of the ahnost-symmetrical supermolecules that exhibited pronounced CD signals, (b) The absolute configuration of knot-type molecule 28 was determined by TDPPP (time-dependent Pariser-Parr-Pople) calculation of the CD spectrum of a fully optimized AMI geometry.

The geometry of the water molecuJe was not altered from its free water values. The N - H - 0 distances for the base (a) and acid (b) forms of hydrogen bonding were optimized for each supermolecule while the 0 - distances (c) and (d) were... 

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