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From: |
yu han <wildheart6143@...24...> |

Date: |
Tue, 21 Nov 2017 12:21:24 +0800 |

Hi!

It’s my first time to write my question to you with this method. I don’t know if the method is right. And if there is any trouble with you, I am sorry.

My problem is about a force of two group. In simple terms, my model is that the middle layer of polymer chains is pressed with two fcc cryatal. What I need to calculate is the pressure between layer and the upper crystal. The units id LJ,and the command is compute group/group. And I found the force is equal with different size of model in a same load(the load way via the command of “addforce”). In other words, The force output by thermo_style with ADDFORCE command at the same load should increase proportionally. However ,the result is identical.

And then I read the manual of units and group/group command!

The manual say that force in three direction are “extensive”, and extensive quantities are output by SUM/ATOM. What? This is contrary to the group/group command/

In the next I tested the easy model with different size. And I got the same conclusion.

And can you tell me where I have wrong?

The following is my in.test.

units lj

atom_style bond

dimension 3

boundary p p s

special_bonds fene

read_data 30.softall

neighbor 0.4 bin

neigh_modify every 1 delay 1

bond_style fene

bond_coeff 1 30.0 1.5 1.0 1.0

pair_style hybrid lj/cut 2.0

pair_modify shift yes

pair_coeff 1 1 lj/cut 1.0 1.0

pair_coeff 1 2 lj/cut 1.0 1.0

pair_coeff 1 3 lj/cut 1.0 1.0

pair_coeff 2 2 lj/cut 1.0 1.0

pair_coeff 3 3 lj/cut 1.0 1.0

pair_coeff 2 3 lj/cut 1.0 1.0

pair_coeff * 4 none

region lo-fixed block INF INF INF INF INF -11.937

region hi-fixed block INF INF INF INF 11.937 INF

group lo region lo-fixed

group hi region hi-fixed

group boundary union lo hi

group mobile subtract all boundary

group up type 2

group chain type 1

group down type 3

group gvv type 4

group bcchi subtract up hi

group bcclo subtract down lo

group slide subtract all lo

fix ff0 gvv setforce 0 0 0

fix 09 hi addforce 0 0 -10.0

fix 22 lo setforce 0.0 0.0 0

variable temp equal 0.001

fix relax all nve

velocity mobile create ${temp} 482748 rot yes dist gaussian

fix 35 mobile temp/berendsen ${temp} ${temp} 0.2

compute friction33 chain group/group bcchi

compute friction22 chain group/group chain compute friction11 bcclo group/group chain

compute friction44 bcchi group/group hi

compute friction55 bcclo group/group lo

compute friction66 chain group/group up

compute 99 chain stress/atom NULL

compute 100 chain reduce sum c_99[1] c_99[2] c_99[3]

compute 111 bcchi property/atom fz

compute 122 bcchi reduce sum c_111

compute 113 hi property/atom fz

compute 124 hi reduce sum c_113

compute 114 bcclo property/atom fz

compute 125 bcclo reduce sum c_114

compute 222 all temp

thermo_style custom step c_friction33[3] c_friction33[1] c_friction11[3] c_friction11[1]

minimize 1.0e-6 1.0e-8 1000 10000

thermo 100

timestep 0.005

dump 1 all custom 100 ./dumpaddforce/dump*.

run 100000

**Follow-Ups**:**Re: [lammps-users] compute group/group in a different model***From:*Steve Plimpton <sjplimp@...24...>

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