# Re: Proposed Feature in Targeted MD

From: V.Ovchinnikov (ovchinnv_at_mit.edu)
Date: Mon Jan 31 2011 - 15:14:15 CST

Dear David,

I have tinkered with the restrained version of TMD that is in NAMD2.6 a
couple of years ago, and I think I have a version that performs a
double-sided TMD using two orientation structures, as you describe
(although I was not using it to compute work, just to look possible
paths). If you give me a couple of days to dig up the relevant files, I
can send them to you off-the-list. Not quite sure how the TMD 2.7 source
code is different from TMD 2.6, but I would be surprised if the
differences were large. (One could polish the code and submit to the
developers for inclusion later, too, if the community thinks its
useful.)

Regards,
Victor

On Mon, 2011-01-31 at 13:12 -0600, Minh, David D. wrote:
> A minor change to targeted MD can make it possible to calculate
> potentials of mean force in the region between two structures.
>
>
> Right now, the code allows targeting toward one final structure, with
> the potential given by the following equation,
> U_{TMD} = \frac{1}{2} \frac{k}{N} [ RMS(t) - RMS^*(t) ]^2
> http://www.ks.uiuc.edu/Research/namd/2.7/ug/node41.html
>
>
> It should be relatively easy to modify targeted MD and have the option
> of targeting the transition between two structures. The potential
> would be,
> U_{TMD} = \frac{1}{2} \frac{k}{N} [ DRMS(t) - DRMS^*(t) ]^2,
> with DRMS(t) = RMS_1(t) - RMS_2(t), and RMS_1 being the RMSD from
> structure 1 and RMS_2 being the RMSD from structure 2, and DRMS^*(t)
> being a time-dependent function. This bias was used in,
> NK Banavali and B Roux. Free Energy Landscape of A-DNA to B-DNA
> Conversion in Aqueous Solution. J. Am. Chem. Soc. 127(18), 6966-6876
> (2005).
> K Arora and CL Brooks. Large-scale allosteric conformational
> transitions of adenylate kinase appear to involve a population-shift
> mechanism. PNAS 104(47), 18496-18501 (2007).
> and probably other papers.
>
>
> Using the theory described in,
> G Hummer and A Szabo. Free energy reconstruction from nonequilibrium
> single-molecule pulling experiments. PNAS 98(7), 3658-61 (2001).
> one could calculate the PMF along the DRMS coordinate from a set of
> TMD trajectories. The theory described in,
> DDL Minh. Multidimensional Potentials of Mean Force from Biased
> Experiments along a Single Coordinate. J. Phys. Chem. B 111,
> 4137-4140 (2007).
> can be used to calculate the PMF along other coordinates, or 2D PMFs.
> The theory in,
> DDL Minh and AB Adib. Optimized Free Energies from Bidirectional
> Single-Molecule Force Spectroscopy. Phys. Rev. Lett 100(18), 180602
> (2008).
> DDL Minh and JD Chodera. Optimal estimators and asymptotic variances
> for nonequilibrium path-ensemble averages. J. Chem. Phys. 131(13),
> 134110 (2009).
> would be useful for combining data for the transition from structure 1
> to structure 2 and the reverse process from structure 2 to structure
> 1.
>
>
> Actually, it is possible to reconstruct PMFs with the current TMD
> code, but I think the PMF between two structures would be more
> interesting.
>
>
> I'd like to give this a try. Would any NAMD developers be interested
> in making this modification? Any thoughts?

This archive was generated by hypermail 2.1.6 : Mon Dec 31 2012 - 23:19:45 CST