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--- exercises:2016_uzh_cmest:faq [2016/11/30 13:11] – tmueller
+++ exercises:2016_uzh_cmest:faq [2020/08/21 10:15] (current) – external edit 127.0.0.1
@@ Line 7: / Line 7: @@
 You can find all available MOLOPT basis sets in ''BASIS_MOLOPT'', which is located in the folder ''$CP2K_DATA_DIR'' (after loading the CP2K module).
-===== When and how do I run CP2K in parallel? =====
+===== How do I run CP2K in parallel? =====
 The command ''cp2k.sopt'' is the single-process variant of CP2K.
-To run CP2K in parallel, you have to use a different executable named ''cp2k.popt'' and prefix that with ''mpirun -n 8'' to run it on 8 CPUs in parallel. So, a complete command line would then read:
+To run CP2K in parallel, you have to use a different executable named ''cp2k.popt'' and prefix that with ''mpirun -n 8'' to run it on 8 CPUs in parallel. So, a complete command line to run CP2K on 8 CPUs would then be:
 <code>
-mpirun -n 8 cp2k.popt -i yourinput.inp -o youroutput.out
+$ mpirun -n 8 cp2k.popt -i yourinput.inp -o youroutput.out
 </code>
+===== How do I obtain a charge density difference? =====
+In [[exercises:2016_uzh_cmest:basic_electronic_structure|Exercise 2]] a series of ''*.cube'' files was already produced for MOs.
+To obtain the electronic charge density in a ''*.cube'', file the following snippet of input file can be used (see also the [[https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/DFT/PRINT/E_DENSITY_CUBE.html|reference manual]]):
+<code>
+&FORCE_EVAL
+  &DFT
+    &PRINT
+      &E_DENSITY_CUBE
+      &END E_DENSITY_CUBE
+      ...
+    &END PRINT
+    ... ...
+  &END DFT
+&END FORCE_EVAL
+</code>
+The [[tools:cubecruncher|cubecruncher]] tool can be used to obtain charge density differences. A precompiled executable is available on the server ''tcopt3'', its path being ''/users/scaravat/bin/cubecruncher.x''.
+The basic usage to obtain a charge density difference is:
+<code>
+$ /users/scaravat/bin/cubecruncher.x -i input.cube -o output.cube -subtract subsystem.cube
+</code>
+===== How do I build a slab model for a surface? =====
+E.g. by using [[http://jp-minerals.org/vesta/en/|VESTA]].
+A tutorial is available [[http://hjklol.mit.edu/content/building-slab-geometries-catalysis-vesta|here]].
+Vesta comes preinstalled on ''tcopt3'', provided that you load the proper module.
+<code>
+$ module load vesta
+</code>
+===== How do I obtain the stress tensor? =====
+You have to add the following snippet to the input file:
+<code>
+&FORCE_EVAL
+  ...
+  STRESS_TENSOR ANALYTICAL
+  &PRINT
+    &STRESS_TENSOR
+    &END STRESS_TENSOR
+  &END PRINT
+  ...
+&END FORCE_EVAL
+</code>
+<note tip>Your calculation should be set up in such a way that forces are being calculated:
+e.g. ''RUN_TYPE'' set to ''GEO_OPT'' or ''ENERGY_FORCE''; instead ''RUN_TYPE ENERGY'' won't work!</note>
+===== My calculation does not converge =====
+==== ... and I use MULTIPLE_UNIT_CELL ====
+  * first make sure that you actually need ''MULTIPLE_UNIT_CELL''. In case you want to do band structure calculation, you most definitely do **not** want it
+  * if you are sure that you need it, make sure that it is specified two times: once in the [[https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/SUBSYS/CELL.html#MULTIPLE_UNIT_CELL|''CELL'' section]] and a second time in the [[https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/SUBSYS/TOPOLOGY.html#desc_MULTIPLE_UNIT_CELL|''TOPOLOGY'' section]], otherwise your calculation may or may not converge