Intermolecular interactions water dimer

The R12 methods have been applied to larger systems. For example, the structure and energetics of crystalline tri-hydrated tricyclic oithoamide, [10]annulene, small water clusters, " intermolecular interactions (He dimer, HF dimer, benzene... Ne), " and ferrocene have been investigated. As a representative example, the computation of a quantitatively accurate six-dimensional PES for the HF dimer is discussed in Section 6.2. 

All of the differences between the MK and RVS methods tend to zero as the intermolecular separation is increased, and so serious differences are expected to arise only for strongly interacting systems. To demonstrate this point. Table 4 presents a comparison between the MK and RVS analysis for H2O dimer and OC-BH3 calculated at the RHF/6-3H-G(d,p) level of theory.In the relatively weakly interacting water dimer there are small differences between the two methods. Notably, the two methods differ with regard to which water molecule contributes most to the polarization energy. The difference in the two methods is much more pronounced for the OC-BH3 system. For the MK analysis, the dominant term is mix, so this theoretical approach is for all practical purposes invalid. The RVS analysis is much more robust, presumably owing to its inclusion of Pauli repulsion. 

Hess O, Caffarel M, Huiszoon C, Claverie P (1990) Second-order exchange effects in intermolecular interactions. The water dimer. J Chem Phys 92 6049... 

Another important question deals with the intramolecular and unimolecular dynamics of the X-—RY and XR -Y- complexes. The interaction between the ion and molecule in these complexes is weak, similar to the intermolecular interactions for van der Waals molecules with hydrogen-bonding interactions like the hydrogen fluoride and water dimers.16 There are only small changes in the structure and vibrational frequencies of the RY and RX molecules when they form the ion-dipole complexes. In the complex, the vibrational frequencies of the intramolecular modes of the molecule are much higher than are the vibrational frequencies of the intermolecular modes, which are formed when the ion and molecule associate. This is illustrated in Table 1, where the vibrational frequencies for CH3C1 and the Cr-CHjCl complex are compared. Because of the disparity between the frequencies for the intermolecular and intramolecular modes, intramolecular vibrational energy redistribution (IVR) between these two types of modes may be slow in the ion-dipole complex.16... 

Table 4.2. Calculated properties of the water dimer. The interaction energy (Eint) in kcal/mol, the intermolecular distance (RQO) in A. 

The SCF-MI BSSE free method does not take into account dispersion forces, connected to electronic intermolecular correlation effects. By using the SCF-MI wave function as a starting point, however, a non orthogonal BSSE free Cl procedure can be developed. This approach was applied to compute intermolecular interactions in water dimer and trimer the resulting ab initio values were used to generate a new NCC-like potential (Niesar et al, 1990). Molecular dynamics simulation of liquid water were performed and satisfactory results obtained (Raimondi et al, 1997). 

High-resolution spectroscopic experiments provide a detailed experimental information on the shape of the intermolecular potential in the attractive regions. Recent improvements in supersonic beams and new laser techniques increased dramatically the sensitivity and resolution in the near-infrared region and opened to high-precision measurements the difficult far-infrared region. The latter development made it possible to investigate directly intermolecular vibration bands which are very sensitive probes of the shape of intermolecular potentials. The new spectroscopic techniques provide a lot of accurate data on interaction potentials for atom-molecule complexes, as well as on more complicated systems such as the HF, ammonia or water dimers. 

The hydrogen-bonded water dimer is without any doubt the most used system to study intermolecular interactions, be it from the QM [34,72] QM/MM [13,26,31,32,40,52,108], or MM [25,42,45,48,50,72] perspective. In the past we have also used it to show that the DRF model indeed gives static and response potentials that are as good as, e.g., SCF calculations [74,137], Of course, if this is the case, it allows for arbitrary separation of the total system into different subsystems, which can then be arbitrarily described at the QM or MM level e.g., for a simple system like the water dimer, one may treat both monomers at the QM level, one monomer at QM and the other at MM, or both monomers at MM. Hence, we may go from the computationally expensive fully QM to QM/MM and to MM, without significant loss of accuracy. Alternatively, we can do MD simulations at the MM level, take snapshots from them and submit these to QM/MM (or QM) calculations to obtain UV-Vis spectra, excitation energies, NLO properties, etc., for the solute in solvent, i.e., sequential MD. 

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. 

---