CHARMM Development Project Forums User Discussion & Questions Molecular Dynamics How to increase external force linearly with time

Hello all,

I noticed that there is an option in pull force command to increase force linearly with respect to time. The issue is that if I invoke the command ' pull force 50 xdir 0 ydir 0 zdir -1 switch 3 sfor 0' before dynamic command, then simulation ceased with 'Linearly time-varying switching has to be invoked'. I understand that the pull has to be invoked with dynamics if I want to increase force linearly, but I just don't know how to do it. I tried:

DYNA cpt leap reSTRT NSTEP 200000 TIMESTEP 0.001 -
IPRFRQ -1 IHTFRQ 0 IEQFRQ 0 NTRFRQ 10000 -
IUNREA 91 IUNCRD 92 iunwrite 93 IASVEL 1 KUNIT 20 -
NPRINT 1000 NSAVC 5000 IHBFRQ -1 imgfrq -1 INBFRQ -1 -
PCONst pmass 1000 pgamma 20 PREFerence 1.0 -
ILBFRQ 1000 RBUFFER 0.0 TBATH 298.15 -
langevin FIRSTT 298.15 FINALT 298.15 TSTRUCT 298.15 TWINDH 0.0 TWINDL 0.0 -
pull force 50 xdir 0 ydir 0 zdir -1 switch 3 sfor 0

Is this the correct way?

I've always used PULL prior to dynamics, but have not tried the time varying option.

What does CHARMM tell you when try that?

Why use Langevin when Hoover is a superior thermostat?

Thanks for reply. I'm using c42b2 version. I used Langevin because I intended to reproduce a literature result simulated by Langevin dynamics. Since force is large for my system, I tried time varying option as suggested in doc. By merging pull command with dyna command, I did not get any error or warning. But PULL energy term is missing in the output.


DYNA> 190000 20390.00000 -969.14465 1835.64860 -2804.79325 298.36576
DYNA PROP> 40.51937 -953.41268 1873.69964 15.73197 -353.24357
DYNA INTERN> 765.31161 818.62664 141.14061 1112.67421 43.24865
DYNA EXTERN> 661.86294 -661.57667 0.00000 0.00000 0.00000
DYNA IMAGES> -338.54527 -1362.13148 0.00000 0.00000 0.00000
DYNA EWALD> 209.44001 -8243.52481 4078.55425 0.00000 0.00000
DYNA PRESS> 1084.60823 -849.11251 -3500.04955 1040.83874 22418.48166
DYNA XTLE> -953.88994 47.32859 0.32697 599.81947
DYNA LRCor> -29.87394 -59.73750

missing (DYNA POLAR: EPOLAR PULL)

PULL is a separate command (ie, it is not an option to the dynamics command).
Usage:
PULL ....
DYNA ....

Several pull-commands may be given prior to the dynamics command:
PULL ...
PULL ...
DYNA ...

See cons.info (or cons.doc), and testcases c25test/pull.inp, c34test/pull_switch.inp

Thank you for your help. I found energy command after the pull command is the issue.

In what way is the energy command an issue?

The energy command (putting after pull command with time varying option) will cause 'Linearly time-varying switching has to be invoked' error.

Is it possible to self-define a time varying force, and implement it in the pull command. For instance, if a force has time dependence of (t^2-2000*t), then:

calc force= (@MDstep)E2-2000*(@MDstep)E2

pull force @force xdir ydir zdir select all end

dynamic command
.....

OK, but a time-dependent force is not so meaningful for the time-independent energy command.
You would have to implement a new functional form in the source code, energy/epull.F90.