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--- exercises:2015_pitt:hfx [2015/03/04 12:26] – [2nd task: PBE0-D3 water] vondele
+++ exercises:2015_pitt:hfx [2015/03/05 09:01] – [Auxiliary Density Matrix Methods (ADMM)] vondele
@@ Line 123: / Line 123: @@
 ===== 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 ?
 ===== Auxiliary Density Matrix Methods (ADMM) =====
+ADMM is an approach to mitigate the cost of HFX for large basis sets. In particular, if MOLOPT basis sets are used, standard HFX becomes too expensive (CP2K can not deal efficiently with highly contracted AOs). In ADMM, an ''AUX_FIT_BASIS_SET'' is introduced, which is used to create an auxiliary density matrix (ADM) by projection. HFX is evaluated for this ADM, while the error introduced by using an ADM is corrected for with a GGA exchange functional.
+==== 4rd task : introduce ADMM ====
+Make the following changes:
+  * insert and additional line ''BASIS_SET_FILE_NAME BASIS_ADMM'' (and copy that file from cp2k/data as needed).
+  * insert for each ''&KIND'' a line ''AUX_FIT_BASIS_SET cFIT3''
+  * insert a secion ''&AUXILIARY_DENSITY_MATRIX_METHOD''
+<code>
+    ! use ADMM
+    &AUXILIARY_DENSITY_MATRIX_METHOD
+      ! recommended, i.e. use a smaller basis for HFX
+      ! each kind will need an AUX_FIT_BASIS_SET.
+      METHOD BASIS_PROJECTION
+      ! recommended, this method is stable and allows for MD.
+      ! can be expensive for large systems
+      ADMM_PURIFICATION_METHOD MO_DIAG
+    &END
+</code>
+<note> In this tutorial we combine ADMM using a very small basis set (cFIT3) with a small primary basis (DZVP-GTH), so gains are small at best (and the results not very accurate). ADMM is most useful with good quality primary basis sets, such as e.g. MOLOPTs</note>
+<note> For CP2K 2.7, a [[|new library of ADMM basis sets]] will be made available.</note>
+Run the input, what's the ''HOMO - LUMO gap'' ?
 ===== Required files =====