Thank so much Justin for the help, 
I am going to reinstall gromacs 5 and try, I decided to install gromacs 4.6.5, 
and instead of a flat  bottom potential , uses position restraint in x and y , 
in order to get the pmf of a drug passing throughout of an ion channel , I 
don't know if this could affect that much due to this bias(do you think so?) , 
but If I have time try to get both using gromacs 5 , 

Thanks again 

Andrés Ortega 
Ing. Electrónica 
Universidad del Valle 

El 7/04/2014, a las 20:45, "Justin Lemkul" <[email protected]> escribió:

> 
> 
> On 4/4/14, 10:48 AM, Andres Ortega Guerrero wrote:
>> Thank you Justin,
>> 
>> I will appreciate the help with this doubt, of the reference group, that is 
>> not 0 , is 26 in the index file,
>> or the appropiate definition of the reference group thanks
> 
> I want to revisit this because I am now working through it myself.  What was 
> not immediately clear is that these indices do not, in fact, have anything to 
> do with the order of the index file.  Instead, the indices correspond to the 
> groups used in pulling.
> 
> Since the pull code is now more general, such that multiple reaction 
> coordinates can now be defined (and thus more than one reference group), one 
> must specify the number of reaction coordinates (pull-ncoords), how many 
> groups total are involved in any sort of reaction coordinate (pull-ngroups), 
> and then identify which of those groups are participating in each of the 
> reaction coordinates.
> 
> The simplest case is to use the biasing potential along one reaction 
> coordinate with two groups.  Therefore:
> 
> pull-ngroups = 2
> pull-ncoords = 1
> pull-coord1-groups = 1 2
> pull-group1-name = refgroup
> pull-group2-name = pullgroup
> 
> I have that much working at this point.  One could, in theory, then extend 
> the situation to have two reaction coordinates between 3 or 4 groups, i.e.
> 
> pull-ngroups = 3
> pull-ncoords = 2
> pull-coord1-groups = 1 2
> pull-coord2-groups = 1 3
> pull-group1-name = refgroup
> pull-group2-name = pullgroup1
> pull-group3-name = pullgroup2
> 
> I assume that will work (famous last words, but I have no time or need to 
> test that setup at the moment).  What it is intended to do is apply a biasing 
> potential between groups 1 and 2, and 1 and 3 simultaneously.
> 
> -Justin
> 
> -- 
> ==================================================
> 
> Justin A. Lemkul, Ph.D.
> Ruth L. Kirschstein NRSA Postdoctoral Fellow
> 
> Department of Pharmaceutical Sciences
> School of Pharmacy
> Health Sciences Facility II, Room 601
> University of Maryland, Baltimore
> 20 Penn St.
> Baltimore, MD 21201
> 
> [email protected] | (410) 706-7441
> http://mackerell.umaryland.edu/~jalemkul
> 
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