Re: Forces of same structure gotten from openmx are a lot different from that from vasp ( No.1 ) 
 Date: 2019/12/18 09:47
 Name: T. Ozaki
 Hi,
The two codes, VASP and OpenMX, use different basis functions and pseudopotentials. Thus, it is difficult to expect that the two codes give an exactly equivalent result. After getting the optimized structure by one of the codes, the optimized structure cannot be optimum for the other code, and vice versa. This is what you saw. Even with the single code, VASP or OpenMX, the optimum structure depends on computational parameters such as quality of basis functions, choice of pseudopotential, and how the real and kspaces are discretized. So, once you change the numerical parameters, you should observe residual forces even for a structure optimized by the same code but with a set of different parameters.
To properly calculate the phonon dispersion using the finite difference method, one needs to keep all the computational parameters in a single code for all the calculations to avoid any numerical noise. Also, rather severe computational parameters have to be chosen to avoid numerical artifacts such as eggbox effect on the energy surface.
We have implemented a code to calculate phonon dispersion using the finite difference method, and obtained results well compared to published ones. http://www.openmxsquare.org/workshop/meeting15/Phonon_OpenMX.pdf http://tozaki.issp.utokyo.ac.jp/meeting16/OMXYTLee2016Nov.pdf
Also, please notice that ALAMODE has been interfaced with OpenMX. https://alamode.readthedocs.io/en/latest/ which may provide a way to calculate phonon dispersions via OpenMX.
Regards,
TO

Re: Forces of same structure gotten from openmx are a lot different from that from vasp ( No.2 ) 
 Date: 2019/12/25 01:27
 Name: Zuzhang Lin <linzz16@mails.tsinghua.edu.cn>
 Dear T. Ozaki
Thank you for your suggestions. I have used ALAMODE and solved the problems. I found the dfference I got before may be due to unsuitable orbital bases I chose.
Best wishes Zuzhang Lin

