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--- exercises:2018_uol_school:converging_cutoff [2018/05/31 10:10] – [Analysis] mwatkins
+++ events:2018_summer_school:converging_cutoff [2020/08/21 10:15] (current) – external edit 127.0.0.1
@@ Line 7: / Line 7: @@
 This exercise is similar to the previous one, but uses a setup and system more typical of CP2K usage. We will use a system of 32 H<sub>2</sub>O water molecules within a periodic box. Here is the input template:
-<code>
+<code cp2k input_template.inp>
 &GLOBAL
   PRINT_LEVEL MEDIUM
@@ Line 91: / Line 91: @@
 Compared to the Si example, this is a larger system, we are using the OT optimizer in a good setup for a small to medium insulating system:
-<code>
+<code cp2k>
     &SCF
       SCF_GUESS RESTART
@@ Line 109: / Line 109: @@
 and we are also saving the forces on the atoms
-<code>
+<code cp2k>
   &PRINT
     &FORCES
@@ Line 194: / Line 194: @@
 <note>**Task**
-If you like, have a look at the BASIS_MOLOPT file (in the data directory, or online [[https://sourceforge.net/p/cp2k/code/HEAD/tree/trunk/cp2k/data/BASIS_MOLOPT|here]]) and see how the exponents change across the periodic table.
+If you like, have a look at the BASIS_MOLOPT file (in the data directory, or online [[https://sourceforge.net/p/cp2k/code/HEAD/tree/trunk/cp2k/data/BASIS_MOLOPT|here]]) and see how the exponents change across the periodic table
 </note>
+The convergence is largely dominated by the calculation of the gradient terms in a GGA functional (compare a simulation with LDA to the PBE used here). The evaluation of these terms on the grids are demanding, and very dependent on the functional.
+<code>
+    &XC
+      &XC_FUNCTIONAL PBE
+      &END XC_FUNCTIONAL
+      &XC_GRID
+        ! defaults
+        XC_SMOOTH_RHO NONE
+        XC_DERIV PW
+      &END XC_GRID
+    &END XC
+</code>
+For BLYP functional some smoothing needs to be applied. The smoothing may also converge forces more rapidly than the default settings, but at the expense of modifying the functional slightly.
+<note>**TASKS**
+compare to the previous calculation, but using a smoothing section in the XC section.
+<code>
+    &XC
+      &XC_FUNCTIONAL PBE
+      &END XC_FUNCTIONAL
+      &XC_GRID
+        XC_SMOOTH_RHO NN50
+        XC_DERIV NN50_SMOOTH
+      &END
+    &END XC
+</code>
+compare the convergence of LDA and BLYP to PBE.
+<code>
+&XC_FUNCTIONAL PADE # or BLYP
+&END XC_FUNCTIONAL
+</code>
+</note>
+<note tip>
+Also change the psuedo potential to the appropriate functional.
+<code>
+    &KIND O
+      BASIS_SET DZVP-MOLOPT-SR-GTH-q6
+      POTENTIAL GTH-PADE-q6
+    &END KIND
+</code>
+PADE is a synonym for LDA.
+</note>