addforce and variable

Hi all,

Sorry to bother again.

I want to add a force to molecules in a region,

This is the input script:

units real
atom_style full
boundary p p p


group water type 1 2

bond_style harmonic
bond_coeff 1 450 0.9572
angle_style harmonic
angle_coeff 1 55 104.52
pair_style lj/cut/coul/long 12.0

kspace_style pppm 0.0001
special_bonds lj/coul 0.0 0.0 0
neighbor 2.0 bin
neigh_modify delay 0 one 2000 check yes

timestep 1.0
thermo_style custom step temp etotal

thermo_style custom step temp etotal press

thermo_modify flush yes line one

thermo 100

fix 2 water nvt temp 300.0 300.0 100.0

velocity water zero linear

region forcing block INF INF INF INF 25 35 units box
variable P equal 2460 (pressure I need, in the unit atm)
variable press_conv_factor equal 0.1439 (convert pressure in the unit ‘atm’ to force/square in the unit kcal/(moleAngstrom))
variable N_in_region equal count(water,forcing)/3
variable force equal v_P
fix 3 water addforce 0.0 0.0 -v_force region forcing

run 500000

But, the force can not be added to the atoms in the region ‘forcing’. Only if I change the comand '‘fix 3 water addforce 0.0 0.0 -1 region forcing’, it works. But, the output value ‘v_force’ is nonzero.

I don’t know how to solve the problem and I’m not sure if I need to change the unit from ‘atm’ to ‘kcal/(mole*angstrom)’