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Re: [lammps-users] Doubt wrt pair style lj/cut/coul/dsf
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Re: [lammps-users] Doubt wrt pair style lj/cut/coul/dsf

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Tue, 5 Sep 2017 13:48:23 -0400

On Tue, Sep 5, 2017 at 12:58 PM, Ana Silveira <anita_silveira@...8...> wrote:
Dear LAMMPS developers,

For pair styles which require a kspace solver, such as lj/cut/coul/long, the special neighbors 1-2, 1-3, and 1-4 are included in the neighbor list. The short-range term q[i]*q[j]*erfc(alpha*r)/r is computed for all pairs with r <= rc, including these special neighbors. However, whenever factor_coul < 1, a calculation is done so that the correct interaction is recovered afterwards when kspace includes the long-range term. Thus,

      prefactor    short-range      special neighbor            kspace
    ------^------   ------^------         ---------^-------            ------^-----
    (q[i]*q[j]/r)*[ erfc(alpha*r) - (1 - factor_coul)  +  erf(alpha*r) ]

This procedure works because erfc(x) + erf(x) = 1. For instance, interaction will fade out if factor_coul = 0.

What I do not understand is why the same procedure is done for lj/cut/coul/dsf and related pair styles, which do not involve a kspace solver and the term prefactor*erf(alpha*r) will not be added afterwards.  In the compute() routine of lj/cut/coul/dsf, the line if (factor_coul < 1.0) forcecoul -= (1.0-factor_coul)*prefactor serves to exclude those interactions which should not have been considered initially. However, they are subtracted without performing any shifting/damping. I’m afraid there is an inconsistency there. Am I right?.

​no. the kspace term is implicit in coul/dsf or coul/wolf. the purpose of this special treatment for special neighbors​ is that in those cases you must exclude the *full* coulomb interaction to be consistent since you cannot exclude them correctly in the kspace calculation, regardless of whether you do that one explicitly or implicitly.

Moreover, I wonder if it is correct to apply any shifting/damping to the special-bond coulombic interactions. To the best of my knowledge, none of the force fields commonly employed in molecular dynamics includes shifting or damping in their parameterization procedures. The 1-4 coulombic interaction is an important part of a dihedral model, so that the fitted parameter values often reflect the presence of such interaction.

​of course not. that is why there is the special treatment in those pair styles. with the code how it is currently in LAMMPS, you always exclude (or scale) the same interaction strength, i.e. the undamped coulomb.



Thank you,

Ana Silveira

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Dr. Axel Kohlmeyer  akohlmey@...12...24...
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.