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 12, 13, and 14 are included in the neighbor list. The shortrange 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 longrange term. Thus, prefactor shortrange 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 = (1.0factor_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 specialbond 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 14 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
