# [gmx-users] REMD Showing Zero Exchange Probability

Ligesh Lichu lichuligesh at gmail.com
Fri Jul 20 14:25:36 CEST 2018

```I have tried an exchange every 2 ps. That is every 1000 steps.

On Fri, Jul 20, 2018 at 5:34 PM, Smith, Micholas D. <smithmd at ornl.gov>
wrote:

> How frequently are you trying to exchange?
>
> ===================
> Micholas Dean Smith, PhD. MRSC
> Post-doctoral Research Associate
> University of Tennessee/Oak Ridge National Laboratory
> Center for Molecular Biophysics
>
> ________________________________________
> From: gromacs.org_gmx-users-bounces at maillist.sys.kth.se <
> gromacs.org_gmx-users-bounces at maillist.sys.kth.se> on behalf of Ligesh
> Lichu <lichuligesh at gmail.com>
> Sent: Friday, July 20, 2018 4:39 AM
> To: gmx-users at gromacs.org
> Subject: [gmx-users] REMD Showing Zero Exchange Probability
>
> Dear all,
>     I have performed REMD for a system containing Protein, Reline, Urea and
> Water in the temperature range 290 to 450 K consist of 16 replicas out of
> 47 replicas generated by REMD temperature generator. But after the MD
> simulation the exchange probability is zero. I have used position
> restraints for reline, urea and protein. Is there any chance that position
> restraints  cause the exchange probability to be zero?  I have one more
> query that, the REMD temperature generator produced around 45 to 54
> replicas for my system in the required temperature range. But I have only
> 80 processors to do the job, So is it necessary to choose the consecutive
> temperature replicas given by the REMD temperature generator or I can skip
> some temperatures in between?
>
> If I am using the equation *Ti = T0 exp (k* i)*, what determines the value
> of 'k'  how it affects the exchange probability? How can I choose the value
> of 'k' for an arbitrary system?
>
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```