<pre>Dear Chris:</pre><pre>Thank you for your help. Maybe I can try it as this. First set x and y fixed by setting compressibility = 0 4.5e-0.5 until the system reaching equilibrium and then release. If it sounds reasonable. I am trying to do it.</pre>
<blockquote style="margin:0 0 0 40px;border:none;padding:0px"><pre>Dear Hugh:</pre><pre></pre><pre>I can't answer your question, but I can address the error message that you see.</pre><pre></pre><pre>Basically, your semi-isotropic pressure coupling allows the surface area of the water-decane</pre>
<pre></pre><pre>interface to get smaller as z increases and x and y decrease. In a system with only water</pre><pre></pre><pre>and decane, this will continue until you have a single file of decane aligned in z.</pre><pre>
</pre><pre>To avoid this, you need to use semi-isotropic pressure coupling but set the compressibility</pre><pre></pre><pre>to zero in the z dimension (or, alternatively set the compressibility to zero in both x and y).</pre>
<pre></pre><pre>This will get rid of the error message, as I see that you have already noted.</pre><pre></pre><pre>So I guess that all I can tell you is that your first option is not going to be possible and</pre><pre></pre>
<pre>that your second option is giving you the wrong value because your surfactants are fighting the</pre><pre></pre><pre>elongation of your box along z, but the effect persists.</pre><pre></pre><pre>Chris.</pre><pre></pre>
<pre>-- original message --</pre><pre></pre><pre>Dear Gromacs users,</pre><pre></pre><pre>I have do simulation of surfactant at oil/water interface for a long time.</pre><pre>But I find it too difficult to get a reasonable interfacial tension. The</pre>
<pre>key question is the pcoupltype.</pre><pre></pre><pre>My system:</pre><pre></pre><pre>Box size: 3.0 * 3.0 * 13</pre><pre>><i> Molecule Number: decane 230, water 2041
</i></pre><pre>><i> pcoupltype = semiisotropic
</i></pre><pre></pre><pre>Gromacs version: 4.5.5</pre><pre></pre><pre>1. When I set compressibility = 4.5e-5 4.5e-5. Surfactant number is 32</pre><pre>. The box will be shrink at x and y. At last simualion will end for a fatal</pre>
<pre>error:</pre><pre></pre><pre>One of the box vectors has become shorter than twice the cut-off length or</pre><pre>box_yy-|box_zy| or box_zz has become smaller than the cut-off.</pre><pre>For more information and tips for troubleshooting, please check the GROMACS.</pre>
<pre></pre><pre>2. When I add the number of surfactants to 40. compressibility = 4.5e-5</pre><pre>4.5e-5. The interfacial tension is -20bar/nm for two interfaces. It is not</pre><pre>reasonable because it should be positive.</pre>
<pre></pre><pre>3. When I compressibility = 0 4.5e-5. Surfactant number is 32. The</pre><pre>interfacial tension is 794bar/nm for two interfaces.</pre><pre></pre><pre>My question is:</pre><pre></pre><pre>What is the right way to simulate for this system? My work confused me too</pre>
<pre>much. Very appreciate for you help.</pre><pre></pre><pre>Hugh</pre></blockquote>