[gmx-users] 3. Re: Strange pullx coordinates (PMF calculations) (Justin Lemkul)

CROUZY Serge 119222 serge.crouzy at cea.fr
Wed Oct 31 17:54:28 CET 2018

Dear Justin

Thanks for your fast answer
OK for the larger group0 COM but as you say my reaction coordinate is the distance between the two COMs - The problem being that in the profile.xvg file the X coordinate reflects the box sizes and thus I can't superimpose the profiles coming from Simulations 1) and 2) - I need to have in the pmf profile X coordinates equal
to the distance between COMs not absolute distance as it is now .. How can I correct the x coordinate in the pmfs so that they represent my real reaction coordinate which is the distance between COMs?  
Isn't it a strange choice to write the absolute coordinates of the COM in the pullx files instead of the real reaction coordinate we all need ?  Now do I have to reread all my trajs, calculate the center of mass of the X coordinate of the COM of DNA (my fixed molecule) and subtract it at each line of the pullx files ... This does not make sense ...

Thanks a lot

On 10/31/18 11:43 AM, CROUZY Serge 119222 wrote:
> Dear gromacs users
> I've been running gromacs for several years, and enjoyed it..
> I've been running PMF calculations for protein DNA interactions without problems until recently:
> Now I'm puzzled with the X coordinates being written in the pullx 
> files (and thus taken as reaction coordinate values in the PMF). This 
> is my problem
> I run two simulations 1)  protein A moving away from DNA                      DNA---A->  along x
>                                         2) same protein A moving away 
> from DNA in the presence of protein B:  B-DNA---A -> along x In 2) A 
> and B interact slightly and I expect to see a slight difference in the 
> profiles for pulling A away In both simulations I'm pulling on center 
> of mass of A away from center of mass of DNA (force along X only)
> My problem is that the x pulling coordinates in simulation 2) (in the pullx files) are around 9 A larger than in simulation 1). Consequence: the profiles are shifted along x by around 9 A. This is not logical to me since my reaction coordinate is distance between com of A and com of DNA which should be the same !!
> It's as if the size of the simulation box (around 10 A larger in 2) to 
> accommodate B ) mattered .. ?!  (I'm running PME with PBC in water...)

Yes, that makes sense. The first column(s) in pullx.xvg are whatever
(x,y,z) components of the group0 COM. If your box is larger, then naturally the position is different. What should not be different is the relative DNA-A distance, which is the actual reaction coordinate. 
Absolute coordinates of any one species don't really matter.



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