Yes, changing the net charge of the system is something that is rather complicated in fact (one can plunge ahead and do it while ignoring the complications, but the results will typically be rather system-size dependent and essentially wrong). For more details refer to the Kastenholz and Hunenberger papers from J. Chem. Phys (2006 or 2007) and references therein. The good news is that for simple spherical ions K&H have worked out the relevant corrections. The bad news is that if you move away from simple spherical ions you've got problems.<div>
<br></div><div>David Mobley</div><div><br></div><div><br><br><div class="gmail_quote">On Fri, Dec 16, 2011 at 6:41 AM, Justin A. Lemkul <span dir="ltr"><<a href="mailto:jalemkul@vt.edu">jalemkul@vt.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im"><br>
<br>
bipin singh wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hello,<br>
<br>
I am willing to study the free energy of binding of a cation (Ca++) to<br>
the protein and I am following the free energy tutorial<br>
provided by Justin<br>
(<a href="http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin/gmx-tutorials/free_energy" target="_blank">http://www.bevanlab.biochem.<u></u>vt.edu/Pages/Personal/justin/<u></u>gmx-tutorials/free_energy</a><br>
).<br>
Please let me know whether the settings for this type of study would<br>
be same as given in the tutorial for ligand-Protein<br>
binding free energy calculation or it need some different approach:<br>
</blockquote>
<br></div>
By decoupling a Ca2+ ion, you are removing 2 charges from the system. I don't know how to properly treat such a case (perhaps someone else can comment), but likely you'll find such topics in the literature. A better approach may be umbrella sampling, but again the literature should point you to reasonable methodology. I'm sure others have dealt with such questions before.<br>
<br>
-Justin<div class="im"><br>
<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
The setting from the ligand-Protein binding free energy calculation<br>
are given as:<br>
<br>
<br>
van der Waals coupling:<br>
<br>
sc-alpha = 0.5 ; use soft-core for LJ (de)coupling<br>
sc-sigma = 0.3<br>
sc-power = 1<br>
couple-moltype = LIG<br>
couple-intramol = no<br>
couple-lambda0 = none ; non-interacting dummy in state A<br>
couple-lambda1 = vdw ; only vdW terms on in state B<br>
<br>
Coulombic coupling:<br>
<br>
sc-alpha = 0 ; soft-core during (dis)charging can<br>
be unstable!<br>
sc-sigma = 0<br>
couple-moltype = LIG<br>
couple-intramol = no<br>
couple-lambda0 = vdw ; only vdW terms in state A (the<br>
previous state B is now A)<br>
couple-lambda1 = vdw-q ; all nonbonded interactions are on in state B<br>
<br>
<br>
</blockquote>
<br>
-- <br></div>
==============================<u></u>==========<br>
<br>
Justin A. Lemkul<br>
Ph.D. Candidate<br>
ICTAS Doctoral Scholar<br>
MILES-IGERT Trainee<br>
Department of Biochemistry<br>
Virginia Tech<br>
Blacksburg, VA<br>
jalemkul[at]<a href="http://vt.edu" target="_blank">vt.edu</a> | <a href="tel:%28540%29%20231-9080" value="+15402319080" target="_blank">(540) 231-9080</a><br>
<a href="http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin" target="_blank">http://www.bevanlab.biochem.<u></u>vt.edu/Pages/Personal/justin</a><br>
<br>
==============================<u></u>==========<div class="HOEnZb"><div class="h5"><br>
-- <br>
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</div></div></blockquote></div><br><br clear="all"><div><br></div>-- <br>David Mobley<br><a href="mailto:dmobley@gmail.com" target="_blank">dmobley@gmail.com</a><br>504-383-3662<br><br><br>
</div>