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## [ESPResSo] Specification of External Potential/Force

**From**: |
Lorenzo Isella |

**Subject**: |
[ESPResSo] Specification of External Potential/Force |

**Date**: |
Tue, 19 Feb 2008 15:11:10 +0100 |

**User-agent**: |
Mozilla-Thunderbird 2.0.0.9 (X11/20080110) |

Dear All,
I think I have a quite unusual problem here.

`I am often (ab)using Espresso to investigate systems and problems quite
``far away from molecular dynamics.
``Now, I would like to consider a system in 3D where you have a pair-wise
``interaction between particles (not relevant right now, but you can think
``of a Lennard-Jones potential) plus an external potential acting on the
``system.
``To fix the ideas, think of a strong harmonic confinement along z, i.e.
``V_z=K_z*z^2 and of a much milder confining potential in the xy plane
``i.e. V_{xy}=K_{xy}(x^2+y^2), with K_z>>K_{xy}.
``This is intended to give rise to rather flat structures along along z
``(together with an interaction potential having a hard-core repulsion).
``(1) Is this doable in Espresso? Can one tune the external
``force/interaction in a similar way as one can do with the tabulated
``interaction potential?
`In the documentation I bumped into:

`[ext_force x value y value z value] An additional external force is
``applied to
`the particle.

`when discussing particle properties, but I am not sure this allows me to
``do exactly what I have in mind [See page 26 of the user guide].
``(2) If I turn off the thermostat, am I using Espresso for deterministic
``molecular dynamics? Am I then removing the "noise" (apart from numerical
``round-off errors) from the simulation?
`
Many thanks
Lorenzo

**[ESPResSo] Specification of External Potential/Force**,
*Lorenzo Isella* **<=**