Force Fields and Numerical Solutions Christian Hedegaard Jensen - within Molecular Dynamics
Outline General Introduction Force Fields Numerical Solutions Test
Outline General Introduction Force Fields Numerical Solutions Test
General Introduction From last time we have that the problem is Force Fields = What is V ? Numerical Solutions = How to solve the equation numerically ?
Outline General Introduction Force Fields Numerical Solutions Test
Force Fields A force field may look like this (taken from [1])
Force Fields Dihedral
Force Fields Lennard-Jones or “Stolen” from [2]
Force Fields Coulomb → (r) model the effect of a solvent. →This can also be modelled explicitly in which case (r) = 1.
Outline General Introduction Force Fields Numerical Solutions Test
Numerical Solutions Predictor-corrector algorithm
Numerical Solutions
Numerical Solutions Verlet algorithm
Numerical Solutions Errors →If you start with slightly perturbed initial conditions the trajectories will diverge from each other eventually. →Fluctuations in energy. For longer time steps the Verlet algorithm is better.
Numerical Solutions Thermostats (If you want to sample at constant temperature) →Example Andersen Thermostat Pick random atom/molecule at intervals Set the velocity so that it is chosen randomly from the Maxwell-Boltzmann distribution This corresponds to introducing collisions with “virtual” heat bath particles.
Outline General Introduction Force Fields Numerical Solutions Test
Test 1. What is the following ? →Coublumb interaction →Lennard-Jones interaction →Verlet interaction
Test The Andersen … is used for what ? → To solve N2 at constant Energy → To calculate forces in the system → To ”solve” N2 at constant Tempreture
Test How is the force on a particle (in one direction) found from the potential ? →
Answers Question 1. Lennard-Jones interaction Question 2. To ”solve” N2 at constant Temperature Question 3.
References [1] N. Rathore et al.; Density of states simulations of proteins; J. Chem. Phys.; v.118 n ; 2002 [2] ki/classes/ch331/protstructure/olunderst andconfo.html M.P. Allen and D. J. Tildesley; Computer Simulations of Liquids; Oxford; 1987