Search results for "force field"

Orthorhombic Phase of Crystalline Polyethylene: A Monte Carlo Study

1996

In this paper we present a classical Monte Carlo simulation of the orthorhombic phase of crystalline polyethylene, using an explicit atom force field with unconstrained bond lengths and angles and periodic boundary conditions. We used a recently developed algorithm which apart from standard Metropolis local moves employs also global moves consisting of displacements of the center of mass of the whole chains in all three spatial directions as well as rotations of the chains around an axis parallel to the crystallographic c-direction. Our simulations are performed in the NpT ensemble, at zero pressure, and extend over the whole range of temperatures in which the orthorhombic phase is experime…

Quantum mechanics-classical molecular dynamics approach to EXAFS

2009

Recently developed approach to the simulation of configuration-averaged EXAFS spectra using the combination of quantum mechanics and classical Molecular Dynamics (MD) methods is presented on the example of the Ti K-edge in SrTiO3 at T = 300 K. The method allows one to significantly reduce the number of fitting parameters required in the EXAFS signal calculation and to account entirely for disorder contributions. We show also that the sensitivity of configuration-averaged EXAFS spectra to the force field model employed in the MD simulations allows one to use them as additional information for the force field parameters fitting.

Box 5: Surface Crystallography Terminology

2009

The crystalline nature of the surface differs from the bulk because atoms on the surface experience a different force field due to unterminated bonds, oxidation by adatoms etc. [1]. Free energy minimisation leads to reconstruction of the surface layer from the bulk by formation of dimers and displacement of atoms from their normal sites.

Quantum chemical calculations of formyl radicals

1975

The ground state (2 A′) of the fluoroformyl radical, FCO, has been investigated by the unrestricted Hartree-Fock method. Pulay's force method for open-shell systems has been applied to calculate the complete quadratic force field and the diagonal cubic force constants. The force relaxation method has been used to determine the equilibrium geometry of FCO: . Adjusting the diagonal force constants to the observed vibrational frequencies of 19F12C16O and taking the values for the off-diagonal force constants from the UHF calculations, the force field of FCO is found to be: . This force field is compared with experiment and with the force fields of HCO, H2CO, and F2CO. The ground state of the F…

Ring inversion pathways of exo- and endo-calix[4]arenes studied by means of the MM3 force field

1995

Abstract The ring inversion pathways of an exo-calix[4]arene 1a and of p-tert-butylcalix[4]arene 2a were investigated by means of the dihedral driver option of the MM3(92) force field. The calculated activation energies of 14.0 and 6.7 kcal mol−1 are in excellent agreement with the experimentally determined value for p-tert-butylcalix[4]arene and the observed flexibility of the exo-calix[4]arene on the NMR timescale.

Inspirations for EO polymer design gained from modeling of chromophore poling by Langevin dynamics

2013

One of the possibilities to create organic molecular material for NLO applications are polymers with dispersed NLO active chromophores. These molecules must be acentrically ordered by applying an external electric poling field. The NLO efficiency depends on dipole moment, molecular hyperpolarizabilities, concentration of the chromophores and external poling field strength. Calculating, from first principles, the extent of the alignment and via this NLO efficiency has proven to be challenging. One approach to solve this problem is pure analytic statistical mechanics treatment, what could be enhanced by Monte Carlo ( MC ) statistical mechanical modelling. The chromophore molecules usually hav…

Fluctuations of the Casimir-Polder force between an atom and a conducting wall

2007

We consider the quantum fluctuations of the Casimir-Polder force between a neutral atom and a perfectly conducting wall in the ground state of the system. In order to obtain the atom-wall force fluctuation we first define an operator directly associated to the force experienced by the atom considered as a polarizable body in an electromagnetic field, and we use a time-averaged force operator in order to avoid ultraviolet divergences appearing in the fluctuation of the force. This time-averaged force operator takes into account that any measurement involves a finite time. We also calculate the Casimir-Polder force fluctuation for an atom between two conducting walls. Experimental observabili…

Dynamical Casimir-Polder force between an atom and a conducting wall

2008

The time-dependent Casimir-Polder force arising during the time evolution of an initially bare two-level atom, interacting with the radiation field and placed near a perfectly conducting wall, is considered. Initially the electromagnetic field is supposed to be in the vacuum state and the atom in its ground state. The analytical expression of the force as a function of time and atom-wall distance, is evaluated from the the time-dependent atom-field interaction energy. Physical features and limits of validity of the results are discussed in detail.

Hybrid QM/MM Molecular Dynamics with AMOEBA Polarizable Embedding

2017

International audience; We present the implementation of a Born-Oppenheimer (BO) hybrid Quantum Mechan-ics/Molecular Mechanics (QM/MM) Molecular Dynamics (MD) strategy using Density Functional Theory (DFT) and the polarizable AMOEBA force field. This approach couples the Gaussian and Tinker suite of programs through a variational formalism allowing for a full self-consistent relaxation of both the AMOEBA induced dipoles and the DFT electronic density at each MD step. As the DFT SCF cycles are the limiting factor in terms of computational efforts and MD stability, we focus on the latter aspect and compare the Time-Reversible BO (TR– BO) and the Extended BO Lagrangian approaches (XL–BO) to th…

The Interaction of Metal Complexes with G-quadruplex DNA

Un approccio computazionale è stato proposto per lo studio dell’interazione di complessi metallici di basi di Schiff con DNA. Nel capitolo 2, è stato investigato il meccanismo di azione di complessi di Nichel(II), Rame(II) e Zinco(II) con B e G-quadruplex DNA. Il G-quadruplex è una conformazione non canonica adottata da particolari sequenze ricche in guanina. Recentemente, è stata dimostrata la sua esistenza in cellule umane, in regioni telomeriche e non telomeriche, ed è stato proposto come un possibile target per una nuova categoria di agenti antineoplastici. I capitoli successivi sono basati su dati raccolti durante due periodi di ricerca all’estero. Nel capitolo 3, basato sugli studi es…