[gmx-users] help for strange linear force in Pulling

[Tom]

Dear Gromacs User,

I am using the pulling to pull two objects apart with Gomacs 4.6.5.
To avoid the issues of half-box size limitation, i used  direction_periodic
to make two objects apart from each more than half-box-size distance.

I just got the linear force: force change with time linearly. This is wrong.
I don't know for the following setting gmx gives linear force.

----------------------------------------------------------------------------------------------
pull                     = umbrella
pull_geometry            = direction_periodic
pull_dim                 = Y Y Y
pull_start               = yes           ; define initial COM distance > 0
pull_ngroups             = 1
pull_group0              = P1
pull_group1              = P2
pull_rate1               = 0.001          ; 0.01 nm per ps = 10 nm per ns
pull_k1                  = 500          ; kJ mol^-1 nm^-2
------------------------------------------------------------------------------------------------------------------

this is the  from pullf.xvg:
-------------------------------------------------------------------------------------------------------------------------
# mdrun_mpi is part of G R O M A C S:
#
# GRowing Old MAkes el Chrono Sweat
@    title "Pull force"
@    xaxis  label "Time (ps)"
@    yaxis  label "Force (kJ/mol/nm)"
@TYPE xy
0.0000  -0
0.0010  0.0001
0.0020  0.0002
0.0030  0.0003
0.0040  0.0004
0.0050  0.0005
0.0060  0.0006
0.0070  0.0007
-----------------------------------------------------------------------------------------------------------
If I switch pull_geometry  = direction, the result makes sense. there would
be no linear force.

Thanks a lot for the help!

Thomas