[Pw_forum] relax vs multiple SCF calculations inconsistency

[Stefano de Gironcoli]

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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