Hello,

I am attempting to do some analysis on outputs predicted by lammps using the DSF potential. The e_coul values predicted in lammps looked a little strange to me, so I came up with a toy case where I could calculate the energy by hand. I simply placed two atoms 4 angstroms apart with no bonded interactions or vdw interactions. I made the charge of one atom 1e, and the other 2e. Using a box of length 1000.0 angstroms, a cutoff of 15.0 angstroms, and a damping parameter of 0.22 angstoms^(-1), and remembering to multiply by the appropriate 332 to convert to kcal/mol, I hand calculate an energy of e_coul = 35.1. However, lammps seems to be calculating -170.6715. Can anyone explain to me this? Is e_coul modified here anywhere besides pair_lj_cut_coul_dsf.cpp?

Numbers for the hand DSF calculation:

term1 = erfc(alpha*r)/r = 0.053
term2 = -erfc(alpha*rcut)/rcut = -2.038

*10^(-7)*

term3 = erfc(alpharcut)/rcut/rcut = 1.3589

term3 = erfc(alpha

*10^(-8)*

term4 = 2alpha/sqrt(pi) * exp(-alpha

term4 = 2

*alpha*rcut

*rcut)/rcut = 3.085*10^(-7)

e_coul = 332*1*2*(term1+term2+(term3+term4)*(4-15)) = 35.1

Here is the data file

LAMMPS data file. CGCMM style. atom_style full generated by VMD/TopoTools v1.2 on Wed Jul 01 22:13:52 EDT 2015

2 atoms

0 bonds

0 angles

0 dihedrals

0 impropers

1 atom types

0 bond types

0 angle types

0 dihedral types

0 improper types

0.0000 1000.00 xlo xhi

0.0000 1000.00 ylo yhi

0.0000 1000.00 zlo zhi