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

From: Ana Silveira <anita_silveira@...8...>
Date: Tue, 5 Sep 2017 16:58:28 +0000

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?.

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.

Thank you,

Ana Silveira