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--- faq:hfx_eps_warning [2016/01/22 12:43] – [What is the EPS_FILTER_MATRIX] ltong
+++ faq:hfx_eps_warning [2023/09/26 12:50] (current) – old revision restored (2023/09/25 19:27) bsertcan
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 ====== I received a warning message about the Kohn Sham matrix not 100% occupied when doing Hartree-Fock or hybrid calculations, what is going on, and what should I do?  ======
+<note important> **Since version 2023.2** The occupancy of the Kohn-Sham matrix depends on whether atomic pairs are neighbours. Two atoms are considered neighbours if their basis functions (Gaussians) are overlapping, which is controlled through the ''EPS_PFG_ORB'' keyword. In large cells, the KS matrix can be sparse because atomic pairs can be further apart. Setting the ''MIN_PAIR_LIST_RADIUS'' to -1 makes sure all atomic pairs within the unit cell are considered neighbours, therefore ensuring a full KS matrix. This is considerably more efficient than reducing ''EPS_PFG_ORB'' to unreasonably small numbers. </note>
 When you do a calculation that involves the calculation of the Hartree-Fock exchange (e.g. calculations with hybrid functionals, including using the ADMM method), you may occasionally encounter the following warning message (more often if ADMM is used):
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 <code>
 The Kohn Sham matrix is not 100% occupied.
-This may result in uncorrect Hartree-Fock results.
+This may result in incorrect Hartree-Fock results.
-Try to decrease EPS_PGF_ORB and EPS_FILTER_MATRIX
+Setting MIN_PAIR_LIST_RADIUS to -1 in the QS section
-in the QS section.
+ensures a fully occupied KS matrix.
 </code>
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 If ''EPS_FILTER_MATRIX'' is already zero, then setting ''EPS_PGF_ORB'' to a smaller value will eventually remove the warning message.  If ''EPS_FILTER_MATRIX'' is not zero, then the first course of action is to reduce it.
-As usual with screening, the typical error you make is on the order of EPS, except the case when the calculation becoems unstable, which yields results that can be essentially unrelated to EPS (e.g. wrong by O(1)). In practice, despite of the warning, if a calculation is stable it should be accurate up to the value of ''EPS_PGF_ORB'' or ''EPS_FILTER_MATRIX''---whichever is larger. You can always check by running a single point calculation with a smaller EPS value, and see if the difference in the total energy is in the order of magnitude as the larger of the EPS value you have tested.
+As usual with screening, the typical error you make is on the order of EPS, except the case when the calculation becomes unstable, which yields results that can be essentially unrelated to EPS (e.g. wrong by O(1)). In practice, despite of the warning, if a calculation is stable it should be accurate up to the value of ''EPS_PGF_ORB'' or ''EPS_FILTER_MATRIX''---whichever is larger. You can always check by running a single point calculation with a smaller EPS value, and see if the difference in the total energy is in the order of magnitude as the larger of the EPS value you have tested.
 The warning message is therefore more for the case when the calculation becomes unstable due to the forced screening of the HF exchange. So if your calculation is unstable, and if you see this warning message, then ''EPS_PGF_ORB'' and ''EPS_FILTER_MATRIX'' are the first places to look.
 Note that by decreasing ''EPS_PGF_ORB'', you will be making the overlap matrix more dense, and correspondingly also the density and KS matrices stored in CP2K.  This will increase your computational cost.  However, for calculations with hybrid functionals the cost of HF exchange term usually dominates and hence the associated cost increase due to more dense overlap, KS and density matrices may not be significant in comparison.