Page - 14 - in Charge Transport in DNA - Insights from Simulations
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TheoreticalBackground
AnMD simulation can be performedwith these equations ofmotion. These are
ordinary differential equations of second order. Numericalmethods are available
to solve these.
2.2.1 Solving theEquationsofMotion
There are several methods for the integration of the equations ofmotion. Rang-
ing fromvery simple ones, like theďŹrst-orderEulermethod, tomore complicated
ones, like the second-order leap-frogmethod, to evenmore complicatedmethods
ofhigherorders. All of these shareone fundamental approach, the introductionof
a time step. This is necessarydue to the fact that the integration canonlybedone
numerically.
The leap-frogmethod [55] is used in thiswork. Here, the positions andvelocities
are evaluated inanalternating fashion.
r(t), v(t+ 1
2 Ît), r(t+Ît), v(t+ 3
2 Ît), r(t+2Ît)...
TheďŹrst step is to calculate thevelocities v (
t+ 12Ît )
v (
t+ 1
2 Ît )
= v (
tâ 1
2 Ît )
+ F(t)
m Ît (2.4)
and thenewatomicpositions are calculated in thenext step
r(t+Ît)= r(t)+ v (
t+ 1
2 Ît )
Ît (2.5)
This algorithmproduces trajectories that are identical to those obtainedwith the
Verlet algorithm[56].
Now, a time stephas to be chosen for thenumerical integration. Thevalue of this
crucial parameter in the simulationdependson two factorsbasically:
⢠First, there are numerical errors due to the second order Taylor expansion.
Contributions inÎt3 andhigherorders areneglected, introducinganerrorof
the same order. To avoid this issue, the step size can be chosen very small,
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Table of contents
- Zusammenfassung 1
- Summary 3
- 1 Introduction 5
- 2 TheoreticalBackground 11
- 3 SimulationSetup 39
- 4 DNAUnderExperimentalConditions 49
- 5 ChargeTransport inStretchedDNA 69
- 6 ChargeTransport inMicrohydratedDNA 79
- 7 AParametrizedModel toSimulateCT inDNA 89
- 8 Conclusion 105
- Appendix 111
- A DNAUnderExperimentalConditions 111
- B CTinMicrohydratedDNA 117
- List ofPublications 137