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faq:hfx_eps_warning

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faq:hfx_eps_warning [2016/01/22 12:43] ltong [What is the EPS_FILTER_MATRIX] |
faq:hfx_eps_warning [2016/01/22 12:45] (current) ltong [What should I do when I see this warning message] |
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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. | 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. | 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. | 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. |

faq/hfx_eps_warning.txt ยท Last modified: 2016/01/22 12:45 by ltong

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