[lammps-users] Virial calculation

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1

Fellow LAMMPS users,

   Is there a way by which I can output the virial from LAMMPS simulation for
each timestep? I am interested in knowing the quantity: SUM(fij.rij) for i<j for
each timestep, where fij is the vector force acting on i due to j and rij is the
distance vector from j to i and '.' represents the dot product between two vectors.

   Since I could not find a direct way to output virial, I thought of
calculating it from the pressure output in log file. For the case of a system of
particles interacting via LJ pair potential, I looked in the pair_lj_cut.cpp, in
the functions PairLJCut::compute(it eflag, int vflag) and
PairLJCut::compute_outer(it eflag, int vflag) the following code is used to
calculate virial:

2

Look in compute_pressure.cpp where all the virial terms
are summed (pair, bond, long-range etc). Then the kinetic
term is added in to get the pressure. You could print
out the virial there.

In the pair routines, the code you mention is b/c
the virial can be computed 2 ways, as Fij Rij or as Fi Ri. The
latter is cheaper, and works when you have ghost atoms
to account for periodic BC, so that's what LAMMPS does when
it can.

Steve