Hi Igor, one obvious difference between your experimental and calculated densities is that the diffraction experiment averages over zero-point and thermal vibrations (what was the temperature ?). The DFT density was (presumably) calculated for the equilibrium structure, i.e., for atoms at rest. For a better comparison, you could run an MD and average the the densities of a series of snapshots. Alternatively, you could repeat the experiment at lower temperature.
Regards, Ulrich. ----- Original Message ----- From: Igor . [mailto:[email protected]] To: [email protected] [mailto:[email protected]] Sent: Tue, 18 Feb 2014 18:51:20 +0100 Subject: [SIESTA-L] Doubt about charge density by rho2xsf and MEM (maximum entropy method) > Hi siesta users,I work with x-ray diffraction in crystals, with the > diffraction data I can generate electron density maps using the technique of > MEM (maximum entropy method), I would like to do some tests and compare > electronic densities generated by MEM and DFT by Utility rho2xsf.I have > chosen a simple material and calculated the electron densities by two > methods. I did it with several materials. The figures of bulk SiO2 and also > the fdf are attached. I used the pseudopotential on the site.I realized that > the electron density generated by DFT have lower values compared to the > isosurface generated by the MEM.What are the reasons for the two results > being different ? The DFT gives the electronic density for the ground state, > how different is the electronic density at room temperature in comparison to > the one ?What can I do to have something more reliable? > > > Best regards --- Max-Planck-Institut für Eisenforschung GmbH Max-Planck-Straße 1 D-40237 Düsseldorf Handelsregister B 2533 Amtsgericht Düsseldorf Geschäftsführung Prof. Dr. Gerhard Dehm Prof. Dr. Jörg Neugebauer Prof. Dr. Dierk Raabe Prof. Dr. Martin Stratmann Dr. Kai de Weldige Ust.-Id.-Nr.: DE 11 93 58 514 Steuernummer: 105 5891 1000 -------------------------------------------------
