Molecular dynamics simulations

Molecular simulations can provide a detailed picture of a desired chemical, physical, or biological process. It has been developed over last 50 years and is being used now to solve a large variety of problems in many different fields. In particular, quantum calculations are very helpful to study small systems at a high resolution where electronic structure of compounds is accounted for. Molecular dynamics simulations, in turn, are employed to study development of a certain molecular ensemble via its development in time and space. Chapter 1 gives a short overview of techniques used today in molecular simulations field, their limitations, and their development. Chapter 2 concentrates on the description of methods used in this work to perform molecular dynamics simulations of cucurbit[6]uril in aqueous and salt solutions as well as metal-isopropanol interface. This is followed by Chapter 3 that outlines main areas in our life where these systems can be used. The development of instruments is as important as the scientific part of molecular simulations like methods and algorithms. Parallelization procedure of the atomistic molecular dynamics program YASP for shared-memory computer architectures is described in Chapter 4. Parallelization was restricted to the most CPU-time consuming parts: neighbour-list construction, calculation of non-bonded, angle and dihedral forces, and constraints. Most of the sequential FORTRAN code was kept; parallel constructs were inserted as compiler directives using the OpenMP standard. Only in the case of the neighbour list the data structure had to be changed. The parallel code achieves a useful speed-up over the sequential version for systems of several thousand atoms and above. On an IBM Regatta p690+, the throughput increases with the number of processors up to a maximum of 12-16 processors depending on characteristics of the simulated systems. On dual-processor Xeon systems, the speed-up is about 1.7. Certainly, these results will be of interest to other scientific groups in academia and industry that would like to improve their own simulation codes. In order to develop a molecular receptor or choose from already existing ones that fits certain needs one must have quite good knowledge of non-covalent host-guest interactions. One also wants to have control over the capture/release process via environment of the receptor (pH, salt concentration, etc.). Chapter 5 is devoted to molecular dynamics simulations preformed to study the microscopic structure and dynamics of cations bound to cucurbit[6]uril (CB[6]) in water and in aqueous solutions of sodium, potassium, and calcium chloride. The molarities are 0.183M for the salts, and 0.0184M for CB[6]. The cations bind only to CB[6] carbonyl oxygens. They are never found inside the CB[6] cavity. Complexes with Na+ and K+ mostly involve one cation, whereas with Ca2+ single- and double-cation complexes are formed in similar proportions. The binding dynamics strongly depends on the type of cation. A smaller size or higher charge increases the residence time of a cation at a given carbonyl oxygen. The diffusion dynamics also corresponds to the binding strength of cations: the stronger binding the slower diffusion and reorientation dynamics. When bound to CB[6], sodium and potassium cations jump mainly between nearest or second-nearest neighbours. Calcium shows no hopping dynamics. It is coordinated predominantly by one CB[6] oxygen. A few water molecules (zero to four) can occupy the CB[6] cavity, which is delimited by the CB[6] oxygen faces. Their residence time is hardly influenced by sodium and potassium ions. In the case of calcium the residence time of the inner water increases notably. A simple structural model for the cations acting as “lids” over the CB[6] portal cannot, however, be confirmed. The slowing of the water exchange by the ions is a consequence of the generally slower dynamics in their presence and of their stable solvation shells. The study of binding behaviour of simple hydrophobic (Lennard-Jones) particles by CB[6] showed that these particles do not bind. A simple test showed that the size of hydrophobic particles in this case is important for a stable encapsulation. Another challenging field of research is the metal-organic interfaces. Particularly, transition metals are more difficult as they form chemical bonds, though sometimes very weak, with a large number of organic compounds. In Chapter 6 a molecular dynamics model and its parameterization procedure are devised and used to study adsorption of isopropanol on platinum(111) (Pt(111)) surface in unsaturated and oversaturated coverages regimes. Static and dynamic properties of the interface between Pt(111) and liquid isopropanol are also investigated. The magnitude of the adsorption energy at unsaturated level increases at higher coverages. At the oversaturated coverage (multilayer adsorption) the adsorption energy reduces, which coincides with findings by Panja et al. in their temperature-programmed desorption experiment (ref. 25). The density analysis showed a strong packing of molecules at the interface followed by a depletion layer and then by an oscillating density profile up to 3 nm. The distribution of individual atom types showed that the first adsorbed layer forms a hydrophobic methyl “brush”. This “brush” then determines the distributions further from the surface. In the second layer methyl and methine groups are closer to the surface and are followed by the hydroxyl groups; the third layer has exactly the inverted distribution. The alternating pattern extends up to about 2 nm from the surface. The orientational structure of molecules as a function of distance of molecules is determined by the atoms distribution and surprisingly does not depend on the electrostatic or chemical interactions of isopropanol with the metal surface. However, possible formation of hydrogen bonds in the first layer is notably influenced by these interactions. The surface-adsorbate interactions influence mobility of isopropanol molecules only in the first layer. Mobility in the higher layers is independent of these interactions. Finally, Chapter 7 summarizes main conclusions of the studies presented in this thesis and outlines perspectives of the future research.