[Pw_forum] relax vs multiple SCF calculations inconsistency

[sarashs]

Ok That does make sense!
Also, today I performed the same calculations with Gaussian 09 and 
3G-STO basis and found the same weird landscape and very close local 
minima/energies which further confirms what you just said. So based on 
that, my system has a loose ZrOSi angle and the energy changes become 
visible only when I make the angle very tight...

Thanks for your help.

Arash


On 2017-04-19 21:02, stefano de gironcoli wrote:
> then the relaxation found a local minimum which is all it is guaranteed
> to find and by exploring the angular dependence you found a better
> minimum or at least a configuration oh lower energy. If the other
> coordinates were not optimized, starting the relaxation from this
> configuration should lead to a better (local) minimum.
> I have no idea whether the 5 mRy=70meV barrier is real or an artifact 
> of
> a noisy landscape due to low cutoff/k-points .
> 
> stefano
> 
> 
> On 19/04/2017 20:16, sarashs wrote:
>> Yes! there is a tiny barrier of 0.005 Ry
>> 
>> Arash
>> 
>> On 2017-04-19 00:20, Stefano de Gironcoli wrote:
>>> Is there an energy barrier between the minimum found by relax and the
>>> one you find varying the angle ?
>>> 
>>> stefano
>>> (sent from my phone)
>>> 
>>>> On 18 Apr 2017, at 21:54, sarashs <sarashs at ece.ubc.ca> wrote:
>>>> 
>>>> They use exactly the same cuttoffs, K-points and everything
>>>> (unfortunately.) but the near equilibrium energies are slightly less
>>>> than equilibrium one. For instance for SiOZr angle at equilibrium I
>>>> get:
>>>> 
>>>> !    total energy              =    -245.22924923 Ry
>>>>       Harris-Foulkes estimate   =    -245.22924923 Ry
>>>>       estimated scf accuracy    <          3.9E-12 Ry
>>>> 
>>>>       The total energy is the sum of the following terms:
>>>> 
>>>>       one-electron contribution =    -715.79581276 Ry
>>>>       hartree contribution      =     369.08543112 Ry
>>>>       xc contribution           =     -57.16353294 Ry
>>>>       ewald contribution        =     158.66552815 Ry
>>>>       Dispersion Correction     =      -0.02086281 Ry
>>>> 
>>>> And for SiOZr angle at equiliberium-25 I get:
>>>> 
>>>> !    total energy              =    -245.23454839 Ry
>>>>       Harris-Foulkes estimate   =    -245.23454839 Ry
>>>>       estimated scf accuracy    <          1.7E-12 Ry
>>>> 
>>>>       The total energy is the sum of the following terms:
>>>> 
>>>>       one-electron contribution =    -739.45020087 Ry
>>>>       hartree contribution      =     380.81843421 Ry
>>>>       xc contribution           =     -57.19470926 Ry
>>>>       ewald contribution        =     170.61520075 Ry
>>>>       Dispersion Correction     =      -0.02327322 Ry
>>>> 
>>>> Which is similar in terms of total energy but slightly lower and
>>>> that's
>>>> weird. Is there anything I can do to force QE to use the same basis
>>>> set
>>>> throughout SCF calculations?
>>>> 
>>>> 
>>>>>> On Tuesday, 18 April 2017 17:10:25 CEST sarashs wrote:
>>>>>> the other SCF's not have higher energy than the
>>>>>> equilibrium angle regardless of them being optimized with a
>>>>>> constraint?
>>>>>> I mean if the structure is originally relax (which it is) then one
>>>>>> expects other near equilibrium structures to have higher energies.
>>>>>> Am
>>>>>> I
>>>>>> wrong there?
>>>>> You are right.
>>>>> 
>>>>> They should be higher, which usually means less negative. As long 
>>>>> as
>>>>> the same
>>>>> pseudopotentials, cutoffs, k-points and everything else is used.
>>>>> 
>>>>> Do they?
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