[lammps-users] Angle Style (Cosine/Squared style)

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1

Hi,
       Looking into the documentation of LAMMPS it seems that the equation displayed for the /cosine/squared /style for angle style in page
http://www.cs.sandia.gov/~sjplimp/lammps/doc/angle_style.html
does not go with what has been coded(square of the difference between cos(theta) terms) in "angle_cosine_squared.cpp" . Thanking in advance for any clarification.

Thanks
Arnab

2

Hi Arnab, there is an error in the documentation. It should read

E = K[cos(theta) - cos(theta_0)]^2

Thanks for catching that.

Naveen

3

Hello,

I have a question on the periodic boundary conditions. I was under the
impression that when using these BCs any size box should behave as a bulk
material. One of the criteria used to determine when a crystal has melted is
the Lindemann criterion, e.g melting occurs when the RMSD of atoms exceed
10% of the lattice constant. However, I have noticed that when the box is
defined smaller than 40 x 40 x 20 nm the RMSD exceeds 10% of the lattice
constant a temperatures much lower than the bulk melting point. Why would
this happen if the simulated material is a bulk and the periodic boundary
conditions make it such that end effects are negligible? I was expecting
that one could define a box of any size with periodic boundary conditions
and get the same results.

Also, I have some problems with the timestep used for the EAM potential.
When I define a free surface the recommended value of 0.001 ps works fine.
However, the same timestep for the case with periodic boundaries on all
sides produces large oscillations on the RMSD when the temperature
increases. I have found that 0.005 ps eliminates these oscillations, but I
would like to know why this is the case.

Thanks
Jaime Sanchez

4

Naveen is correct - the code was right, the doc was wrong.
I just changed the doc page on the WWW page to the
correct form.

Steve