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q: charges? basis sets/potentials validation? Date: 2006/10/16 20:56 Name: Grigory Dear Prof. Ozaki, I'm new to the matherial science calculations, and my background lies in small-molecule gaussian basis set calculations. So please forgice me if my following questions will be somewhat silly. I'm trying to calculate some metal complexes with OpenMX; there is a great PAO and VPS library of yours here on the website. I've read often that one have to validate a pseudopotential (say, for a given cut-off radius) before production use of in on some test systems. So I'm trying now to reproduce with OpenMX some small molecules and reactions for which I have previously used a gaussian basis DFT code. 1) Is it possible to calculate a charged systems with OpenMX? If yes, how to specify it -- by the scf.system.charge variable only or changing occupations in the atoms' specifications as well? ( I've read this http://www.openmx-square.org/forum/patio.cgi?mode=view&no=44 but did'nt fully get it -- is the "automatic" cell size for clusters not necessary optimal for charged species?) 2) is the calculation with OpenMX "size consistent"? I.e., can one calculate reagent A, then reagent B, then its product AB as clusters, separately, and take E(AB)-E(A)-E(B) as the reaction energy? Thank you! -- Grigory

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Re: q: charges? basis sets/potentials validation? ( No.1 ) Date: 2006/11/15 20:37 Name: Vasilii Artyukhov "2) is the calculation with OpenMX "size consistent"? I.e., can one calculate reagent A, then reagent B, then its product AB as clusters, separately, and take E(AB)-E(A)-E(B) as the reaction energy?" To get good accuracy of bond energies, you must ensure that you are using exactly the same bases, pseudopotentials, unit cell size, and the integration grid (scf.energycutoff). Since you're trying to use automated determination of the cell size, you get different cells (and hence, different grids as well) for A, B, and AB. Therefore, you should manually set the same cell size for all three calculations. This will lead to some error cancellation and improve your results. Still, your calculations will suffer from the so-called basis set superposition error. Since you are using different basis sets for the particles (even if these consist of same functions but centered at different points), the error cancellation when subtracting energies is not that precise. To correct this, you should either use dummy atoms to ensure that you have the same bases for all calculations, or use other types of basis sets such as plane waves (which are not available in OpenMX or other O(N) codes). Of course, the same holds for Gaussian bases. Re: q: charges? basis sets/potentials validation? ( No.2 ) Date: 2006/11/18 22:59 Name: Grigory Thank you!

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