Recently, I am studying the organic/water interface interaction by using lammps. In order to solvate the organic molecules fully, the I hope to scale down the coulombic interactions by a factor 500 in this software at the first step. Could anyone tell me how? Thanks in advance.
It can simply be done by knowing that the Coulombic interaction energy is:
U = \sum_{i=1}^{N-1} \sum_{j=i+1}^N k_e q_i q_j / r^2_{ij}
where k_e=1/(4 pi \epsilon_0 \epsilon), with \epsilon_0 permittivity of free space and \epsilon is the dielectric constant, q_i and q_j are the partial atomic charges of atoms i and j, and r_{ij} is their separation.
Thus, you can reduce these interactions by reducing partial charges on atoms (i.e., in your case setting q_i = q_i/\sqrt{500}, for i=1, ..., N) or by increasing \epsilon (typically in explicit solvent simulations \epsilon=1) to, let's say, 500 * \epsilon.
However, you should be careful, because the force field parameters are calibrated (simultaneously) such that to reproduce experimental bulk properties of systems, such as, structural and thermodynamic properties.
You can use a dielectric command: http://lammps.sandia.gov/doc/dielectric.html
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