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Difference in total energy between version 3.6 and 3.7 Date: 2013/08/01 19:18 Name: Bob Schoeters   <bob.schoeters@uantwerpen.be> Dear Prof. Ozaki, before I started using OpenMX 3.7 I started running some tests, to see if I could proceed with the calculations I was doing but now with the new version. I compared the total energy + the forces computed by both versions and saw a difference. Both calculations were done using exactly the same inputfile (so with the same pseudopotentials). The system considered consists of 3 unit cells of a SiGe core-shell nanowire of about 2nm. Version 3.6 gives: |Maximum force| (Hartree/Bohr) = 0.000837613655 ******************************************************* Total Energy (Hartree) at MD = 1 ******************************************************* Uele = -488.416677253263 Ukin = 498.666362793599 UH0 = -20767.418393882697 UH1 = 3.342455266330 Una = -650.232402107206 Unl = 195.596132325449 Uxc0 = -369.494148477977 Uxc1 = -369.494148477977 Ucore = 19890.981590572967 Uhub = 0.000000000000 Ucs = 0.000000000000 Uzs = 0.000000000000 Uzo = 0.000000000000 Uef = 0.000000000000 UvdW = 0.000000000000 Utot = -1568.05255198751138 Version 3.7: |Maximum force| (Hartree/Bohr) = 0.000816178047 ******************************************************* Total Energy (Hartree) at MD = 1 ******************************************************* Uele = -488.415484052568 Ukin = 498.672542744951 UH0 = -20767.419154943644 UH1 = 3.342344175158 Una = -650.236186219205 Unl = 195.598465760279 Uxc0 = -369.488766114339 Uxc1 = -369.488766114339 Ucore = 19890.981590572963 Uhub = 0.000000000000 Ucs = 0.000000000000 Uzs = 0.000000000000 Uzo = 0.000000000000 Uef = 0.000000000000 UvdW = 0.000000000000 Utot = -1568.037930138175 I am aware that the absolute value of the total energy has little meaning, but since I use the same pseudopotential and except some upgrades the same code, I find this strange. The difference in energy looks small in Hartree, but is very large if you are for example interested in formation energies. Could you explain the reason(s) for these differences? Now I am in doubt whether to continue this study using version 3.6 or recalculating everything everything using 3.7, since the speed up is remarkable. Kind regards, Bob Schoeters

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Re: Difference in total energy between version 3.6 and 3.7 ( No.1 ) Date: 2013/08/01 23:25 Name: T. Ozaki Hi, At least, there are two places where methods have been changed for the numerical integration in Ver. 3.7 compared to Ver. 3.6. The first one is how a set of FFT grids is generated based on the keyword 'scf.cutoffenergy'. Please check whether you used the same number of grids along a-, b-, and c-axes for both the calculations. The second one is how the exchange-correlation energy is evaluated. In Ver. 3.7, the Levedev grid for spherical integrals has been introduced for efficiency and accuracy. These changes must actually cause the difference. Of course, you have to use the same version (either Ver. 3.6 or Ver. 3.7) for evaluation of the formation energy so that the error related to the numerical integration can be cancelled out. Regards, TO Re: Difference in total energy between version 3.6 and 3.7 ( No.2 ) Date: 2013/08/02 16:53 Name: Bob Schoeters  <bob.schoeters@uantwerpen.be> Dear Prof. Ozaki, thank you for the quick and clear response. Regards, Bob

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