Get the number of atoms in the dynamic group

LAMMPS LAMMPS Development
1

Dear Lammps users,

I defined a dynamic group in the input. I calculated the peoperty of a dynamic group and oupt put it by
“fix ave/time 10 100 1000”, however I would like to normalize it by the number of atoms in the group.
I wonder how to get the number of atoms or total number of atoms during the simulation.

Thanks

2

You could define an equal style variable where you have access to either property and pass that to your ave/time fix.

3

The property I was trying to calculate is the force between two groups and I would like to normalize the force by the number of atoms in the group. I defined two groups : one is a dyanmic group defined by region and another one is a static group. then I compute the force between those two groups and count the number of atoms in the dynamics group, finally I output the force and number by fix ave/time. I used below commands to do so.

group trapped_ion dynamic sodium region Region_trapped_ion every 10
compute friction_ion0 trapped_ion group/group water kspace yes
variable num_na equal count(trapped_ion)
fix 16 all ave/time 10 100 1000 c_friction_ion0 c_friction_ion0[1] c_friction_ion0[2] c_friction_ion0[3] v_num_na file friction_ion0.txt

I got the decimal number for the force and the number of atoms. I don’t understand why the number of atoms is a decimal number. Correspondingly, I don’t understand what does the force mean as well. For example, if the force is 0.5 and the number of atoms is 0.2. Does it mean the force/per atom is 0.5/0.2=2.5?

Thanks

4

You don’t understand what you are doing at all, yet you are using complex commands without thinking through what they do. For example: why should a property that is an average over a variable (integer) property also be an integer?

I don’t know how to help you if you cannot understand this.