[Pw_forum] 回覆： Re: [Pw_forum] Mg.pz-n-vbc.UPF O.pz-mt.UPF give negative pressure in MgO md calculation

[Lawrence Lee]

dear Nicola,

>>1) please use your affiliation/address in these requests, if you'd like
people like me to answer them (you do not have to, but also I do not
have to answer)

Sorry! But do you mean those lines at the end of the mail, telling who I am and which university I am in? Or any thing to do with the setting of the sent mail? For the former case, I will add it from this time onwards. For the latter, I completely don't know what you refer to. Can you say clearly? I am an internet idiot!  :-)

>>2) the pressure will be zero at the equilibrium lattice parameter -
so please tell us which are the equilibrium lattice parameters of your
LDA and PW91 calculations, and how they differ from what is published in
the literature.

The literature value is somewhere around 7.9 (bohr radius). I cannot exactly recall it. I am doing my md run at 7.8 (very near equilibrium). I tried at least 3 other pp. One is that one I mention (the pw91 one). Another two are self-made. They give 18GPa, 12GPa, 0.7GPa, at the same 7.8 bohr radius of lattice parameter. But using (Mg.pz-n-vbc.UPF & O.pz-mt.UPF) give -160GPa !! A large negative value!

>>3)

Though I am still a freshman, I have been working on MgO for half a year. I have done those you have mentioned. 1)static lattice 0 K calculation to find eqm lattice parameter by minimization on both total energy and pressure. 2) testing on convergence of cutoff energy and k-points. The cutoff for Oxygen is a bit lower than acceptable, but it should be still fine for a rough trial run as that cutoff used in my other 3 calculations using different pp works well (upon fitting the  results of different lattice parameter with Birch-Murnaghan 3rd order, agrees well with the literature value at least in the not too high pressure range).

For safety, I am redoing the run using higher cutoff, because this pp for O is not ultrasoft, but the 3  used previouly by me are all ultrasoft. 

I know you may think that I am just a freshman posting questions without firstly do some detailed testing and thinking. I apologize for giving incomplete information on posting questions. Yet I have done many of those I shall check before asking here. I don't dare saying all of those, but at least those I can think of.

Thank you very very much for giving me the advice :-). I will try to improve.

Lawrence

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Lawrence Lee Shun Hang, The Chinese University of Hong Kong, Hong Kong

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