<div dir="ltr"><div>Hi</div><div><br></div><div>Following what Matt has said, i.e. you might need to use an extremely large cutoff for the q10 pseudoptential of Mg, if you are happy with the GTH-PBE-q2 pseudopotential of Mg with only two valence electrons for your system, you may try the attached DZVP-q2 basis set which I have reoptimised for Mg to be used with the GTH-PBE-q2 pseudopotential. You should be able to get reasonable results with a normal cutoff (e.g. ~400 Ry) using the q2 pseudopotential of Mg.</div><div><br></div><div>SL</div><div><br></div></div><div><br><div class="gmail_quote">On 16 July 2015 at 21:48, Matt W <span dir="ltr"><<a href="mailto:MattWa...@gmail.com" target="_blank">MattWa...@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">Hi Olivia,<div><br></div><div>Mg2+ is nearly pathological if you use the default q10 pseudo and the GPW method. It needs a plane wave cutoff of around 1200 Ry or so to start getting reasonable answers. If you are sticking with non-hybrid functionals, like PBE, then try using the GAPW method (QS/METHOD section) and see if that helps. A cutoff of around 350 Ry should give well converged numbers then.</div><span><font color="#888888"><div><br></div></font></span><div><span><font color="#888888">Matt</font></span><div><div><br><br>On Thursday, July 16, 2015 at 11:41:44 AM UTC+1, Olivia Lynes wrote:<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;padding-left:1ex;border-left-color:rgb(204,204,204);border-left-width:1px;border-left-style:solid"><div dir="ltr">Hi all,<div><br></div><div>I'm having a problem with a few of my geometry optimisations not converging after several thousand steps and having looked at the energy changes through the main output file it's not decreasing but is fluctuating a lot in a certain range. I'm at a loss to why as I've got similar calculations completed with similar input and just a different box size and they have converged without this fluctuation in a few hundred steps.</div><div><br></div><div>I've attached the input, coordination files and the first hundred or so steps of the output file.</div><div><br></div><div>These are geometry optimisations of an Mg 2+ ion in a box of 64 waters, with a cell size of 13.41 angstroms, so the overall system is charged 2. I've used the GTH DZVP basis sets and potentials. </div><div><br></div><div>Having done a cell optimisation on just the 64 waters, we introduced the Mg ion and charged the system. In lieu of being able to do cell optimisations on a charged system, then geometry optimisations are being done at varying box sizes by 10ths of an angstrom as a sort of manual cell optimisation to try to find both a minimum energy structure and a minimum energy box size. </div><div><br></div><div>I've done similar calculations using Ca and Sr and haven't had this issue with any of my jobs, they all converge in a few hundred steps. </div><div><br></div><div>Is this potentially a problem using charged systems? Is it necessary for me to put a counter ion in which would let me do a standard cell optimisation? </div><div><br></div><div>Or is this approach completely wrong and we would be better off doing MD? </div><div><br></div><div>The aim is to do ab initio MD on these systems to look at the coordination of the ion in a bulk of water, so reaching a minimum energy structure and box size isn't essential but it would be nice to have the regular DFT data to back it up. </div><div><br></div><div>Thanks in advance for any insights.</div><div><br></div><div>Olivia</div><div><br></div><div><br></div><div><br></div></div></blockquote></div></div></div></div><div><div>
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