[CP2K:10678] Re: Cp2k convergence test: is it possible that cutoff of 250 Ry has fairly lower total energy than 350 Ry (with same rel_cutoff 60 Ry)?

[Yun An]

Dear Sergey,

Thanks a lot for your reply!

Best regards,
Yun

Sergey Chulkov <sergeya... at gmail.com> 于2018年8月27日周一 上午6:06写道：

> Hi Yun,
>
> The purpose of the convergence test is to find the lowest possible value
> of the plane-wave cutoff that still gives you an accurate total energy. The
> actual accurate total energy may be higher then the "minimal" energy across
> all possible values of the plain-wave cutoff.
>
> In case of sparse real-space grids (small cutoff) there is no uniform
> dependence between the total energy and the value of the plain-wave cutoff.
> The main reason is that different energy components are computed using
> different basis set representations. For example, a smaller plain-wave
> cutoff can potentially give you an underestimated exchange-correlation
> energy, but overestimated electrostatic energy between nuclei and electrons
> which can finally lead to a lower total energy.
>
> For a reasonable guess for the cutoff value you can refer to this page (
> https://www.cp2k.org/faq:cutoff). The relative cutoff of 50 Ry is usually
> a good choice, the relative cutoff of 60 Ry is typically enough in most
> cases.
>
> Best wishes,
> Sergey
>
> On Friday, August 24, 2018 at 3:29:24 PM UTC+1, any... at gmail.com wrote:
>>
>> Dear CP2K developers and users,
>>
>> I did the convergence tests with cp2k 3.0 for hBN system, following the
>> instructions of https://www.cp2k.org/howto:converging_cutoff . The
>> energy values and the NG on grids are as follows. I wonder why higher
>> cutoff values give fairly higher energies, for example, the total energy
>> for cutoff 350 ry is higher than 300 ry, I thought in principle, higher
>> cutoff values could have more stable energy (lower value), at least not
>> that higher than the number with lower cutoffs. But here in my case, it's
>> not, can anybody help me to see why?  Is 250Ry and rel_cutoff of 70 Ry here
>> a good choice?
>>
>> Also, when one starts a calculation,  should the convergence tests always
>> be taken with different values of cut off and rel_cutoff? or it's safe to
>> just use larger values such as (cutoff 400 ry, rel_cutoff 80ry)?
>>
>> Thanks a lot in advance for all your suggestions.
>>
>> Best regards,
>> Yun
>>
>> # Grid cutoff vs total energy
>> # Date: Fri Aug 24 13:38:48 CEST 2018
>> # PWD: /home/Convergence_test
>> # REL_CUTOFF = 60
>> # Cutoff (Ry) | Total Energy (Ha) | NG on grid 1 | NG on grid 2 | NG on
>> grid 3 | NG on grid 4
>>      50.00  -833.3666229006     559488   81536       0       0
>>     100.00  -824.4110369450     457024  180160    3840       0
>>     150.00  -824.3925595993     424576  134912   81536       0
>>     200.00  -824.3899074807     345088  210560   81536    3840
>>     250.00  -824.3979520649     294272  162752  180160    3840
>>     300.00  -824.3920162797   294272  162752  180160    3840
>>     350.00  -824.3858376968    271744  185280  119488   64512
>>     400.00  -824.3841314539    271744  152832  134912   81536
>>     450.00  -824.3831328479     271744  152832  134912   81536
>>     500.00  -824.3833590835     236416  184320  138752   81536
>>
>>
>> # Rel Grid cutoff vs total energy
>> # Date: Fri Aug 24 14:54:23 CEST 2018
>> # PWD: /home/Convergence_test/Rel_cutoff
>> # CUTOFF = 250
>> # Rel Cutoff (Ry) | Total Energy (Ha) | NG on grid 1 | NG on grid 2 | NG
>> on grid 3 | NG on grid 4
>>      10.00  -824.9353843726     24384  212032  184320  220288
>>      20.00  -824.4674485372     117312  176960  162752  184000
>>      30.00  -824.3980791451    236416  184320  138752   81536
>>      40.00  -824.3980825574     271744  152832  134912   81536
>>      50.00  -824.3979520649     294272  162752  180160    3840
>>      60.00  -824.3979520649     294272  162752  180160    3840
>>      70.00  -824.3979509738     345088  210560   81536    3840
>>      80.00  -824.3979509738     345088  210560   81536    3840
>>      90.00  -824.3979509738     420736  138752   79616    1920
>>     100.00  -824.3979509738    424576  134912   81536       0
>>
>>
>> --
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