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--- exercises:2018_uzh_cmest:defects_in_silicon [2018/09/17 12:52] – external edit 127.0.0.1
+++ exercises:2018_uzh_cmest:defects_in_silicon [2020/08/21 10:15] (current) – external edit 127.0.0.1
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 </note>
-For both geometries create a vacancy by removing one Silicon atom, re-calculate the total energy and compare it to the total energy of the intact bulk Silicon minus the single atom energy. What do you observe? Why?
+For both geometries create a vacancy by removing one Silicon atom, re-calculate the total energy and compare it to the total energy of the intact bulk Silicon minus the single atom energy (= vacancy formation energy). What do you observe? Why?
 <note tip>You may have to employ some of the techniques mentioned in [[PDOS|Projected density of states and Band structure for WO$_3$]] to make the calculations converge.</note>
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 Finally we are going to look at the change of the density of states due to the vacancy:
-Alter the input files for the small geometry (the ''silicon8'') with and without the vacancy to print out the projected density of states as shown in [[PDOS|a previous exercise]] and plot the total density of states for both cases. What do you observe when comparing the band gap of the two geometries?
+Alter the input files for the small geometry (the ''silicon8'') with and without the vacancy to print out the projected density of states as shown in [[PDOS|a previous exercise]] and plot the total density of states for both cases, comment. Can you explain why the
+''vacancy'' calculation is harder to converge ?
 Now do a geometry optimization on the ''silicon8'' structure with the vacancy and plot the total density of states on that relaxed structure again. Compare again to the total density of states for the unaltered structure, what do you see?
+Your last task is to compare the total energy of the geometry optimized (with the vacancy) ''silicon8'' structure to that of the standard one minus the energy of a single atom. That is, compute the vacancy formation energy with the relaxed structure and compare it to the one obtained previously. Which of those is the best representation of reality and why ?