Dear vaspmaster and users,
I am an experienced VASP user and I have a question on a topic that has been bothering me for some time, it’s about the ENAUG tag.
I always set ENCUT manually, as in my experience it often needs to be set much larger than the default value, in particular e.g. for accurate calculations on transition metal oxides. I always scale ENAUG to ENCUT as ENAUG=1.4*ENCUT (in combination with PREC=High), as this used to be mentioned in older versions of the VASP manual.
I now read in the recent version of the VASP manual that the ENAUG tag has been deprecated (and that PREC=Accurate should be used rather than PREC=High), however it is not clear to me that the accuracy of the augmentation functions is well taken care of. Sometimes I use ENCUT values that are even larger than the default ENAUG values, and then it does not feel right if I do not set ENAUG manually.
If I enter higher values of ENCUT *without* specifying ENAUG (while using PREC=Accurate), are the augmentation functions also calculated more accurately (more or less scaling with higher values of ENCUT) and can I trust that the augmentation functions are dealt with sufficiently accurately?
Your help or answer is much appreciated.
ENAUG tag deprecated - accuracy augmentation grid?
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vanhuis
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fabien_tran1
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Re: ENAUG tag deprecated - accuracy augmentation grid?
Hi,
Indeed, PREC=Low, Medium and High are "deprecated" (until recently they were indicated as "old", but no change has been made in the code).
In the case of PREC=Normal, Single(N) and Accurate, specifying ENAUG in INCAR has no effect since it won't be used. Nevertheless, this is still possible to control the accuracy for the augmentation functions by using the NGXF, NGYF and NGZF tags (not very convenient). However, also note that the accuracy for the augmentation functions is improved by increasing ENCUT since NGXF, NGYF and NGZF are by default proportional to NGX, NGY and NGZ (see the table at PREC) that are determined by ENCUT.
Indeed, PREC=Low, Medium and High are "deprecated" (until recently they were indicated as "old", but no change has been made in the code).
In the case of PREC=Normal, Single(N) and Accurate, specifying ENAUG in INCAR has no effect since it won't be used. Nevertheless, this is still possible to control the accuracy for the augmentation functions by using the NGXF, NGYF and NGZF tags (not very convenient). However, also note that the accuracy for the augmentation functions is improved by increasing ENCUT since NGXF, NGYF and NGZF are by default proportional to NGX, NGY and NGZ (see the table at PREC) that are determined by ENCUT.
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vanhuis
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Re: ENAUG tag deprecated - accuracy augmentation grid?
Dear Fabian,
Thank you for your reply. I see now that for the PREC=Accurate setting, NGXF is 2*NGX where NGX is a function of ENCUT, consequently the augmentation fine grid will also improve when increasing ENCUT, that is good to know. I don't know how this relates to the NGXF=(16/3)*Gaug that previously worked/works with PREC=High and setting ENAUG by hand, I will run some tests.
Thank you for your reply. I see now that for the PREC=Accurate setting, NGXF is 2*NGX where NGX is a function of ENCUT, consequently the augmentation fine grid will also improve when increasing ENCUT, that is good to know. I don't know how this relates to the NGXF=(16/3)*Gaug that previously worked/works with PREC=High and setting ENAUG by hand, I will run some tests.