<div dir="ltr"><div><div><div><div><div>Dear Dr. Flo,<br><br></div>Thanks a lot for the reply. My main aim of this project is to study the gold clusters on graphene quantum dots (GQDs) and compare the energies of different conformers of gold clusters on these GQDs. I have varied the GQDs from monolayer (290 atoms) to trilayer (830 atoms) in the studies. I have attached the coordinates of GQD+Au cluster for the input which I have given in the previous post (name of the file doesn't represent the original system, though). <br>
<br></div>I took 38*38*38 box to avoid any interaction between the periodic images and I also used Poisson solver to truncate the wavefunction at the edges for the same reason. <br><br></div><b>If it is useful:</b> My main aim is to check which gold conformer is more stable on a GQD. When I checked the difference in the energies (between two conformers) by varying the cutoff from 320 to 500 Ry it didn't change more than 0.01 eV, and hence, I got satisfied with 320 Ry. But, I am also suffering from faster convergence of the calculations. Indeed the convergence issues are becoming more problematic when I am considering bigger systems (i.e. bi-layer or tri-layer GQDs). But, I have also observed that, with the same 320 Ry cutoff, the PBE calculations converged pretty faster.<br>
<br></div>Thanking you for your valuable time,<br></div>Sincerely,<br>Chaitanya Sharma. <br></div><div class="gmail_extra"><br clear="all"><div><div dir="ltr"><span style="color:rgb(204,51,204)"><font color="#000000"><br>
<br><br><br><br><br><br></font>********************************************************</span><br><font><b style="color:rgb(51,51,255)">Chaitanya Sharma,</b></font><br><b><span style="font-family:times new roman,serif">Prof. Bala and Prof. Pati'</span></b>s groups,<br>
<span style="color:rgb(56,118,29)">Chemistry and Physics Materials unit</span>,<br><span style="color:rgb(255,0,0)">JNCASR</span>, BANGLORE,<br>Lab:: (080-2208) <span style="color:rgb(0,0,255)">2581, 2809</span><br><a href="https://sites.google.com/site/sharmasrkcyamijala/" target="_blank">https://sites.google.com/site/sharmasrkcyamijala/</a><br>
<span style="color:rgb(204,51,204)">*********************************************************</span><br></div></div>
<br><br><div class="gmail_quote">On Sat, Mar 8, 2014 at 2:25 PM, Florian Schiffmann <span dir="ltr"><<a href="mailto:flosch...@gmail.com" target="_blank">flosch...@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr">I see the problem. You have a farily big cell (38x38x38) and going to high cutoffs is likely to cause a memory issue. The problem is, BLYP pseudopotetnials need very high cutoffs to be smooth with the FFT's. I don't know what your system looks like exactly but the only way out here as far as I can tell is to use a smaller cell if you can't afford the settings as they are. I don't know what the system looks like (dimensionwise) but it might be possible to use MT and 2D periodic boundaries with a smaller box permitting you to use higher cutoffs. Do you mind sending me a snapshot or position file?<br>
<br>If you don't use the center coordinates keyword with the final geometry the positions should be fine (and optimal at least with the PW basis used but wrong compared to the basis set limit). Furthermore if you want to do energy comparisons (reactions or whatever) you definitely need a better convergence of your PW basis otherwise there will be a position depndent (relative to the gridpoints) error in all your energies.<br>
About the flucuations in the energy. BFGS is very sensitive to the ripples in the PES and only behaves well if they are down to a reasonable magnitude (see above more expensive but better convergence). <br><br>Flo<br></div>
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