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--- exercises:2015_pitt:hfx [2015/03/04 12:16] – [Truncated Coulomb operator] vondele
+++ exercises:2015_pitt:hfx [2015/03/04 12:45] – [Truncated Coulomb operator with long range correction] vondele
@@ Line 119: / Line 119: @@
    * How to detect instabilities due to too aggressive screening.
-Question: What is the HOMO-LUMO gap for this configuration ? How does this compare to the GGA result ?
+Question: What is the HOMO-LUMO gap for this configuration ? How does this compare to the GGA result ? Adjust the fraction of exchange (modify the input in two places!) to 20% and/or 30%, how does this influence the gap ?
 ===== Truncated Coulomb operator with long range correction =====
+Like in the HSE functional, the difference between the operator used for exchange and 1/r, can be accounted for by a special GGA exchange functional. Also for the truncated coulomb operator this is possible, and allows for xc functionals that embed very short range exchange operators only. This can be used to speedup the calculation, while retaining the benefits of HFX. The functional employed in this way smoothly goes from PBE to PBE0 as the range goes from 0 to Infinity.
+==== 3rd task ====
+Add to the &XC_FUNCTIONAL section (i.e. in addition to &PBE) the following section:
+<code>
+       &PBE_HOLE_T_C_LR
+         CUTOFF_RADIUS 2.5
+         SCALE_X 0.25
+       &END
+</code>
+and employ the same ''CUTOFF_RADIUS'' for the ''INTERACTION_POTENTIAL''.
+Rerun the single point energy calculation and note the band gap.
+  * Is such a short range sufficient to have a sizable effect on the band gap ?
+  * is ''HFX_MEM_INFO| Number of cart. primitive ERI's calculated'' very different for calculations with 2.5 and 6.0A truncation radius ?