# [lammps-users] What does evdwl in the DPD package contain?

Hi:

I’m trying to modify the DPD package (since it seems like one of the two pair_styles that communicates velocities) and use this modified dpd package along with LJ using pair_style hybrid/overlay

Reading the pair_dpd.cpp file brings me to this section

dot = delxdelvx + delydelvy + delz*delvz;
wd = 1.0 - r/cut[itype][jtype];
randnum = random->gaussian();

// conservative force = a0 * wd
// drag force = -gamma * wd^2 * (delx dot delv) / r
// random force = sigma * wd * rnd * dtinvsqrt;

fpair = a0[itype][jtype]wd;
fpair -= gamma[itype][jtype]wdwd
dotrinv;
fpair += sigma[itype][jtype]wdrandnum
dtinvsqrt;
fpair = factor_dpdrinv;

f[i][0] += delxfpair;
f[i][1] += dely
fpair;
f[i][2] += delzfpair;
if (newton_pair || j < nlocal) {
f[j][0] -= delx
fpair;
f[j][1] -= delyfpair;
f[j][2] -= delz
fpair;

which seems like the force on each atom being decremented according to the DPD formulation. I change this appropriately.

The next bit, however, I’m not so sure about. The name evdwl seems to suggest a van-der-waals energy, but I’m really not sure what the variable contains. Could someone please explain? Setting it to zero does not cause lammps to crash, but if I compute the pairwise energy using

compute x all pe/atom

the values it produces seem strange.

Many thanks.

if (eflag) {
evdwl = -a0[itype][jtype] * r * (1.0 - 0.5*r/cut[itype][jtype]);
evdwl *= factor_dpd;
}

if (evflag) ev_tally(i,j,nlocal,newton_pair,
evdwl,0.0,fpair,delx,dely,delz);
}

Many thanks.

evdwl does store Vanderwaals energy
The tally function is simply storing it on a global
or peratom basis, depending on how it will
be accessed/needed on that particular timestep,
also accounting for Newton's 3rd law.

Steve