Ulrich Essmann

A smooth particle mesh Ewald method

The previously developed particle mesh Ewald method is reformulated in terms of efficient B‐spline interpolation of the structure factors. This reformulation allows a natural extension of the method to potentials of the form 1/rp with p≥1. Furthermore, efficient calculation of the virial tensor follows. Use of B‐splines in place of Lagrange interpolation leads to analytic gradients as well as a significant improvement in the accuracy. We demonstrate that arbitrary accuracy can be achieved, independent of system size N, at a cost that scales as N log(N). For biomolecular systems with many thousands of atoms this method permits the use of Ewald summation at a computational cost comparable to that of a simple truncation method of 10 A or less.

Effect of the treatment of long‐range forces on the dynamics of ions in aqueous solutions

The goal of the present work is to study the dependence of the limiting ionic mobility of such anions as fluoride, chloride, and bromide in water on the way the long‐range forces are treated in the computer simulations. With this in mind we have performed molecular dynamics computer simulations where the long‐range electrostatic forces were treated using: (a) simple truncation procedure, (b) energy switching procedure, (c) reaction field method, and (d) Ewald summation technique. Our analysis shows that the switching procedure with the short‐range switching function introduces artifacts into the simulations. These artifacts are responsible for the faster decay and oscillations in the velocity autocorrelation function of the ions and therefore for the lower value of the diffusion coefficients.

Dynamical Properties of Phospholipid Bilayers from Computer Simulation

We present the results of a 10-ns molecular dynamics simulation of a dipalmitoylphosphatidylcholine/water system. The main emphasis of the present study is on the investigation of the stability over a long time and the dynamic properties of the water/membrane system. The motion of the lipid molecules is characterized by the center of mass movement and the displacement of individual atom groups. Because of the slow movement of the headgroup atoms, their contributions to the dipole potential vary slowly and with a large amplitude. Nevertheless, the water molecules compensate the strong fluctuations and maintain an almost constant total dipole potential. From the lateral displacement of the center of masses, we calculate the lateral diffusion coefficient to be Dlat = (3 +/- 0.6) x 10(-7) cm2/s, in agreement with neutron scattering results. The rotational motion is also investigated in our simulations. The calculated value for the rotational diffusion coefficient parallel to the molecular long axis, D = (1.6 +/- 0.1) x 10(8) s-1, is in good agreement with the experiment.

Is there a second critical point in liquid water

The supercooled and stretched regions of the phase diagram of simulated liquid water are investigated by calculating the equation of state of the ST2 and TIP4P pair-potentials. We find that simulated water does not display a re-entrant spinodal and that the projection of the density maximum line in the plane of pressure and temperature becomes positively sloped on stretching. The well-known anomalous behavior of supercooled water is tentatively associated with the existence of an inaccessible critical point. Evidence is presented that suggests the association of this new critical point with the transition between low density and high density amorphous solid water. We show how the observed transformation behavior of the two forms of amorphous solid water can be explained in terms of a first order phase transition, via a consideration of the limits of metastability associated with this kind of transition, and support this interpretation with simulations of the amorphous solid. We therefore propose a phase diagram which accounts for the behavior of both liquid and amorphous solid water.

The structure of water at platinum/water interfaces Molecular dynamics computer simulations

Abstract Three pairs of simulations are considered in this work: two simulations of the water lamina between Pt surfaces at zero external electric field, two simulations with the external field turned on and having a value of 10 10 V/m and finally two simulations with the field having the value of 2 × 10 10 V/m. In every pair of simulations we considered the cases of water lamina embedded between two Pt(100) surfaces and between two Pt(111) surfaces. Our simulations demonstrate that water next to a Pt surface is profoundly influenced by this surface. When the surface is uncharged, we observed that the water structure is distorted from its bulk structure to distances of ∼ 1nm from the surface. The simulations show that the water layer adsorbed on a surface is in a solid-like state. The second and the third layer of water are liquid, but the orientational distribution in these layers show preference towards the orientations that are characteristic for hexagonal ice. When we turn on a homogeneous electric field of value 10 10 V/m water molecules reorient, aligning their dipoles along the field. When the field is doubled compared to the previous value, a dramatic change is seen in the density profile, indicating that water in the lamina has undergone crystallization.

Molecular dynamics simulation of vapor deposited amorphous ice

We report a molecular dynamics simulation of the vapor deposition of amorphous ice on a cold surface. We compare the obtained structure with neutron scattering data of high and low density amorphous ice formed by pressure induced transformation of crystalline ice. The structure of our vapor deposited ice is intermediate between these two, although closer to high density amorphous ice. Its radial distribution functions resemble the results of a simulation of cluster formation in the gas phase as well as of a recent neutron scattering experiment on vapor deposited amorphous ice. The occurrence of an intermediate structure is also in agreement with a recent electron diffraction study. Structural differences are discussed in terms of the hydrogen‐bond network. The amorphous surface layer is deeply fissured, suggesting a high porosity of vapor deposited ice.

The role of water in the hydration force - molecular dynamics simulations

To understand the contri- bution of water to the repulsive force acting between phospholipid membrane molecules we performed molecular dynamics computer simu- lations on lamellar systems of phospholipid bilayers in water. Four simulations were performed. Two simulations were done on dilauroyl- phosphatidylethanolamine (DLPE) in water systems and two on dipal- mitoylphosphatidylcholine (DPPC) in water systems. The simulations differed by the amount of water per phospholipid headgroup. From the simulations we concluded that even at the hydration limit the headgroups of the opposing membranes come in to close proximity, so that they are separated by only one or two water layers. Since the water structure in the first solvation shell is distinctly differ- ent from bulk water, the hydration force is likely to be due to the solva- tion layer of water around the head- groups, which are separated by up to two water layers, rather than due to a long range perturbation of the water structure. We also observed that different solvation patterns exist for water around the phosphatidylcho- line (PC) and phosphatidyletha- nolamine (PE) headgroups. We propose that this solvation pattern is connected to the difference in the hydration of DPPC and DLPE membranes.

Novel Features in the Equation of State of Metastable Water

The thermodynamic properties of two commonly used water pair-potentials, ST2 and TIP4P, are calculated from molecular dynamics simulations. In particular, the properties of supercooled and stretched states are found, yielding a determination of the equation of state (EOS) in this region. A unexpected feature in the calculated EOS appears at the lowest temperatures T, in the form of an inflection in the liquid phase isotherms of pressure P versus density ρ. This EOS does not exhibit a re-entrant liquid spinodal, and also predicts that the line of density maxima in the phase diagram has a maximum in T as a function of P. These same behaviors are observed in simulations of SiO2 (using a rigid ion potential), indicating that such behavior may be a generic feature of liquids forming tetrahedral networks. The form of the calculated EOS for water suggests that a critical point may occur in the liquid state phase diagram at lower T. Simulations of the amorphous solid states of water confirm the possibility that this critical point may be the end-point of a line of first order phase transitions, itself originally proposed from experimental observations, separating low and high density amorphous ice.