Differences

This shows you the differences between two versions of the page.

--- exercises:2015_pitt:hfx [2015/03/04 12:45] – [Truncated Coulomb operator with long range correction] vondele
+++ exercises:2015_pitt:hfx [2020/08/21 10:15] (current) – external edit 127.0.0.1
@@ Line 144: / Line 144: @@
 ===== 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 [[https://sourceforge.net/p/cp2k/code/HEAD/tree/trunk/cp2k/data/BASIS_ADMM_MOLOPT|new library of ADMM basis sets]] will be made available.</note>
+Run the input, what's the ''HOMO - LUMO gap'' ?
+===== Chasing charge localization in liquid water =====
+The combination of truncated exchange and ADMM results in the most effective way to run AIMD with hybrid functionals. In some systems the difference between GGA DFT and hybrids is very large. One such systems is liquid water after ionization (i.e. charge +1), where only with hybrids the expected species (OH radicals) are formed. See [[doi>10.1063/1.3664746]].
+==== 5th task : ionized water ====
+<note>This task is optional, and can be done near the end if time is available. </note>
+adapt the admm input for water to reflect the ionized state:
+<code>
+    ! Charge and multiplicity
+    LSD
+    CHARGE 1
+    MULTIPLICITY 2
+</code>
+because the system is electronically very difficult initially, we'll reduce the convergence threshold ''EPS_SCF 1.0E-5'' (twice).
+Note that the ''WFN_RESTART_FILE_NAME'' must point to a GGA calculation of the same charge and multiplicity (do this calculations first).
+Run single point energy calculations varying the fraction of exchange from 0.25 to 0.50,  does the mulliken spin population reproduce Fig. 2 in [[doi>10.1021/ct1002225]] ?
+For the fraction 0.5, run AIMD for about 50-100fs (if time permits), what happens with the water molecule on which the spin was localized ? Do you results agree with [[doi>10.1063/1.3664746]] ?
 ===== Required files =====