Water dimers

The described overall procedure and the final form of the potential are applicable to any complex of an atom and a linear molecule. They also may be generalized to more complex systems, composed of polyatomic monomers, as exemplified in the next Section by the water dimer. 

How efficient is the described representation of the ArCC 2 potential To answer this question the above PES along with a few empirical potentials have been used to derive a number of properties, such as the ground vibrational state and dissociation energy of the complex, ground state rotational constants, the mean square torque, the interaction second virial coefficients, diffusion coefficients, mixture viscosities, thermal conductivities, the NMR relaxation cross sections, and many others [47]. Overall, the ab initio surface provided very good simulations of the empirical estimates of all studied properties. The only parameters that were not accurately reproduced were the interaction second virial coefficients. It is important that its performance proved comparable to the best empirical surface 3A of Bohac, Marshall and Miller [48], This fact must be greeted with satisfaction since no empirical adjustments were performed for the ab initio surface. 

As a second model potential we shall briefly discuss the PES for the water dimer. Analytical potentials developed from ab initio calculations have been available since the mid seventies, when Clementi and collaborators proposed their MCY potential [49], More recent calculations by dementi s group led to the development of the NCC surface, which also included many-body induction effects (see below) [50]. Both potentials were fitted to the total energy and therefore their individual energy components are not faithfully represented. For the purposes of the present discussion we will focus on another ab initio potential, which was designed primarily with the interaction energy components in mind by Millot and Stone [51]. This PES was obtained by applying the same philosophy as in the case of ArCC 2, i.e., both the template and calibration originate from the quantum chemical calculations, and are rooted in the perturbation theory of intermolecular forces. 

Compared to the ArCC 2 complex there is an important new factor - the electrostatic 

In the water-dimer potential of Ref.[51], each water molecule has three interaction sites centered on atoms. The long-range electrostatic energy assumes the functional form 

Source From Lii and AlUnger, Copyright 2008 by the American Chemical Society. Reprinted by permission of the American Chemical Society. 

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Leforestier C, Braiy L B, Liu K, Eirod M J and Saykaiiy R J 1997 Fuiiy ooupied 6-dimensionai oaiouiations of the water dimer VRT states with a spiit Wigner pseudo-speotrai approaoh J. Chem. Phys. 106 8527-44... 

The intennolecular forces between water molecules are strongly non-additive. It is not realistic to expect any pair potential to reproduce the properties of both the water dimer and the larger clusters, let alone liquid water. There has therefore been a great deal of work on developing potential models with explicit pairwise-additive and nonadditive parts [44, 50, 51]. It appears that, when this is done, the energy of the larger clusters and ice has a nonadditive contribution of about 30%. 

Hg. 6.13 Minimum energy structure for water dimer with TIP3P model... 

Compare your results to the experimental values of -34.0 2 kcal mol for the lithium reaction and -3.6 .5 kcal mol" for the water dimer reaction. Use the same model chemistry as in Example 8.2 B3LYP/6-311+G(2df,2p) // B3LYP/6-31G(d). 

Optimizing water dimer can be challenging in general, and DFT methods are known to have difficulty with weakly-bound complexes. When your optimization succeeds, make sure that you have found a minimum and not a transition structure by verifying that there are no imaginary frequencies. In the course of developing this exercise, we needed to restart our initial optimization from an improved intermediate step and to use Opt=CalcAII to reach a minimum. 

Y ater dimer distance variation provides a sequence of structures for water dimer at different nonbonded OH distances. Plot energy (vertical axis) vs. nonbonded OH distance (horizontal axis). What is the optimum distance How much energy is required to increase this distance by 10% How much is required to reduce the distance by 10% Are the two distortion energies about the same magnitude If not, explain why not. 

One after the other, examine methanol dimer and acetic acid dimer. Do the hydrogen-bond lengths in these systems differ significantly from the optimum distance in water dimer Are the hydrogen-bond angles in these compounds significantly different from those in water dimer Rationalize your results. 

One additional important reason why nonbonded parameters from quantum chemistry cannot be used directly, even if they could be calculated accurately, is that they have to implicitly account for everything that has been neglected three-body terms, polarization, etc. (One should add that this applies to experimental parameters as well A set of parameters describing a water dimer in vacuum will, in general, not give the correct properties of bulk liquid water.) Hence, in practice, it is much more useful to tune these parameters to reproduce thermodynamic or dynamical properties of bulk systems (fluids, polymers, etc.) [51-53], Recently, it has been shown, how the cumbersome trial-and-error procedure can be automated [54-56A],... 

The water dimer is probably the most intensively studied intermolecular hydrogen bonded system of all. Hence, ample theoretical and experimental data is available for this system,... 

Table 12-1. Deviation in the computed Rq 0 distance of the water dimer [A] from the experimental Re value of 2.952 A.

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