# Molecular Dynamics simulations Surface interaction energy

Hey,
I've simulated a solid metal cluster of gold fcc100
And a surface Platinum in fcc111 using a classical mechanical approach.
Do you think i could get the surface free energies(the ones to use in determining the Wulff shape) from periodic boundary conditions and if so how? Im really confused, i need to make an approximation of the height and width of the melted particle so i would like the energy in the UP/z direction.

The gold cluster is cubic at the initial state btw.

Surface energies can be obtained using a slab model in which you orient the surface of interest to be normal to z-axis (for example). You apply periodic boundary conditions in x and y but in z-direction you have two options. Either still using periodic boundary conditions but leave a vacuum gap above the surface of a thickness > 1nm (a convergence test is needed here) or if you are using classical potentials you can actually remove the periodicity in z and use a vacuum boundary condition in this direction. IN that case
Esurf=(Eslab-Ebulk)/2 , The factor of two because you have two exposed surfaces.

The nanoparticle/ cluster approach you described above is not common and indeed may not always be useful to calculate the surface energy beacause:
1- All the faces of the cluster should represent the same surface. This is very hard to achieve unless the material has a crystal structure of high symmetry.
2- One may need a very large cluster to guarantee convergence.

Hi again,
Thank you for the clear answer.
I just want to estimate the energies for determining how high and wide the cluster will be at some state.
I simulate an actual surface with an on it resting cluster, and not a potential difference, so it is a bit more coumbersome i think.

But how will i determine the wulff shape from the initial state, i have only 6 faces and to me it looks like it will be a triangle(3d). Do you have any litterature to recommend, this stuff is mabye not so much at my level(undergraduate).

Thanks again
Edit:
Btw. it is the contanct energy between cluster and surface which is interesting

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