Hello all,

I am using compute group/group command to calculate force from Cu atoms to

c atoms since I am using pair potential between two. However for Si and C,

I am using tersoff potential, Compute group/group command doesn't work.

How to calculate force on one group due to another group if there is no

pair potential between two groups. Kindly suggest.

for tersoff, you can try compute force/tally from the USER-TALLY package.

you will get a warning, but tersoff is compatible with tallying forces,

only the virial tally is not completely covered by USER-TALLY for tersoff.

another approach would be to record the trajectory and then use rerun on

it, but have only the desired interacting potentials defined and the rest

to non using a hybrid style. then you should get the same result from

simply doing compute reduce for fx fy fz.

please see below a small test example of two tersoff si spheres set up to

bouncing into each other.

note how the scalar (total force) from compute force/tally is the same as

the reduced fx component on the left sphere and the reduced tallied x

component of the pairwise forces. for the right sphere, the sign is

inverted. now, if there was an additional component in the system, the

reduced fx, fy, fz components would be different, but the tallied not,

since they would only be accumulated for forces between the two pairs.

hence, you would need to use rerun with disabling unwanted interactions

plus compute reduce or try compute force/tally, but the latter will not

work for a

units metal

atom_style atomic

atom_modify map array

boundary m m m

# diamond unit cell

variable a equal 5.431

lattice custom $a &

a1 1.0 0.0 0.0 &

a2 0.0 1.0 0.0 &

a3 0.0 0.0 1.0 &

basis 0.0 0.0 0.0 &

basis 0.0 0.5 0.5 &

basis 0.5 0.0 0.5 &

basis 0.5 0.5 0.0 &

basis 0.25 0.25 0.25 &

basis 0.25 0.75 0.75 &

basis 0.75 0.25 0.75 &

basis 0.75 0.75 0.25

region myreg block 0 8 &

0 4 &

0 4

create_box 2 myreg

region left sphere 2.0 2.0 2.0 1.2

region right sphere 6.0 2.0 2.0 1.2

create_atoms 1 region left

create_atoms 2 region right

group left region left

group right region right

# choose potential

pair_style tersoff

pair_coeff * * SiC.tersoff Si Si

mass * 28.06

velocity left set 0.2 0.0 0.0

velocity right set -0.2 0.0 0.0

compute ft left force/tally right

compute fl left reduce sum fx fy fz c_ft[1] c_ft[2] c_ft[3]

compute fr right reduce sum fx fy fz c_ft[1] c_ft[2] c_ft[3]

fix 1 all nve

timestep 0.001

#dump 1 all movie 100 movie.mpg type type

thermo_style custom step temp pe vol c_ft c_fl[*] c_fr[*]

thermo 1000

run 10000