[gmx-users] MD after equilibration phase

[Justin A. Lemkul]

bipin singh wrote:
> Also, if we give continuation=yes in mdp file and use input as pdb file 
> as input instead of gro file, grompp never complains....I don't no how 
> it reads velocities from pdb file (as no velocities are present in pdb 
> files). Ideally it should complain that no velocities found in input 
> file....
> 

Again, the "continuation" keyword has nothing to do with velocities.  If you 
have "gen_vel = no" and you provide a .pdb file with no velocities, then you do 
not preserve velocity information.  Whatever the initial forces are govern the 
resulting motions.  In any case, you do not preserve the previous simulation 
conditions.

-Justin

> On Fri, Apr 6, 2012 at 12:42, Peter C. Lai <pcl at uab.edu 
> <mailto:pcl at uab.edu>> wrote:
> 
>     On 2012-04-06 11:05:51AM +0400, James Starlight wrote:
>      > Dear Gromacs users!
>      >
>      >
>      > I have small question about order of the runs and input data.
>      >
>      > Ussually I do 2 equilibration phases and subsequent productive
>     phase in the
>      > conditions wich are equal to the last equilibration phase ( e.g
>     often this
>      > is npt ).
>      >
>      > In the second equil.mdp and md.mdp there is option
>      >
>      > continuation             = yes
>      >
>      > which means that there have been previous phases of the
>     simulation from
>      > wich  coordinates and velocities should be taken.
>      >
>      > As I understood the coordinates is taken from .gro file but from
>     what file
>      > the velocities must be providen ? Does it .cpt checkpoint file from
>      > previous run? In some cases I've forgotten to define -t npt.cpt
>     for my MD
>      > run providing only coordinates in GRO file, topology and md.mdp
>     but I have
>      > not seen any errors in such simulation due to absence of that
>     .cpt and
>      > GROMPP never remind me of the absense of this file. What exactly
>     is in that
>      > .cpt file and from wich source the velocities from equilibration
>     phase are
>      > taken ?
> 
>     continuation = yes is telling LINCS that it is a continuation and
>     it should not attempt to refit the constrained bonds on the first pass.
> 
>     The coords, velocities, state, and box information are taken either
>     from the
>     cpt file or you can specify the previous .trr and it will take the
>     last frame
>     from that and use it. If you used the output gro file without -t, then
>     it will take coordinates and velocities from the .gro but the
>     problem there
>     is the limited precision (3 decimal points for each).
> 
>     --
>     ==================================================================
>     Peter C. Lai                    | University of Alabama-Birmingham
>     Programmer/Analyst              | KAUL 752A
>     Genetics, Div. of Research      | 705 South 20th Street
>     pcl at uab.edu <mailto:pcl at uab.edu>                     | Birmingham AL
>     35294-4461
>     (205) 690-0808                  |
>     ==================================================================
> 
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> 
> 
> -- 
> -----------------------
> /Regards,/
> Bipin Singh
> 

-- 
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Justin A. Lemkul
Ph.D. Candidate
ICTAS Doctoral Scholar
MILES-IGERT Trainee
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080
http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin

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