[CP2K-user] [CP2K:13757] Re: H2 single molecule AIMD

[Lucas Lodeiro]

Hi Marcella an Travis,

I try both advice. To improve the forces, I increase the cutoff from 350 to
700 Ry and rel_cutoff from 40 to 60 Ry, and decrease the eps_scf (inner and
outer scf) from 1.0E-6 to 1.0E-8. This does not solve the rotational
problem, but the pump is lower than the no-improve input. The system shows
equal the two fake peaks but the "distance" between them is smaller,
and approaches
the corrected trajectory with the true one peak.
For the second, I set angular velocity tolerance equal to 1.0E-11. In this
case the rotation is off along all the trajectory, just a little
temperature decreasing along the trajectory (17 K in 40 ps).

Firstly I want to thank you for your disposition and advice. Secondly, I
have some questions.
In the case of improving the forces, as I am using a time step of 0.25 fs,
I think the cutoff and eps values are the main parameters to improve the
forces, isn't it? Do you expect a better improvement?
In the case of angular velocity tolerance, a value of 1.0E-11, is a too
little one? There is a typical value to balance the angular movement and to
subtract the lesser amount of kinetic energy?

Regards

El mar., 11 ago. 2020 a las 3:12, Marcella Iannuzzi (<marci... at gmail.com>)
escribió:

> Hi Lucas,
>
> I would rather try to improve the accuracy of the integration of the
> equations of motion.
> The error in the forces is probably too large to guarantee the right
> behaviour of the conserved quantities.
> Regards
> Marcella
>
>
> On Monday, August 10, 2020 at 6:59:57 PM UTC+2 Lucas Lodeiro wrote:
>
>> Hi Marcella,
>> I try the MD with periodic and non periodic conditions. When ANGVEL_ZERO
>> was applied for a non periodic cell, the simulation starts with zero
>> angular velocity, but the numerical inaccuracies pumps some angular
>> velocity at the first steps... and the molecule starts rotating.
>> As you mention, for a diatomic molecule it is not possible to apply any
>> constraint... in all cases it will be not physical.
>> Is there a way to fix the rotational movement?
>>
>> El lun., 10 ago. 2020 a las 6:03, Marcella Iannuzzi (<ma... at gmail.com>)
>> escribió:
>>
>>> The translational and rotational degrees of freedom are subtracted,
>>> hence fixing 4 more degrees of freedom  by constraints you end up with a
>>> negative number of DoF, which is obviously not physical.
>>> Are you using PBC? The ANGVEL_ZERO keyword is ignored with PBC
>>> Regards
>>> Marcella
>>>
>>>
>>>
>>> On Sunday, August 9, 2020 at 10:26:49 PM UTC+2 Lucas Lodeiro wrote:
>>>
>>>> Hi all!
>>>>
>>>> I am exploring single molecule AIMD (H2 in this case) for different
>>>> purposes. The main problem that I face is temperature/rotational modes. I
>>>> try to run a AIMD using NVE and NVT ensembles, but there are some problems.
>>>> For an NVE ensemble, the system starts rotating with a conserved
>>>> angular moment, even if I set equal to zero the total angular
>>>> moment... This disrupt the linear movement of H2 bond oscillation. The
>>>> rotation couples with the bond and generates a fake two frequency
>>>> oscillator. If, after the AIMD, I correct the trajectory to a zero total
>>>> angular moment for all frames (simple for diatomic homonuclear molecule,
>>>> but not so easy for polyatomic molecules), the oscillator recovers the
>>>> single frequency motion.
>>>> For an NVT  ensemble, the thermalization is so hard for few atoms...
>>>> and the system is very bad behaved.
>>>>
>>>> Now I try an NVE ensemble with constrain MD, fixing the Y and Z
>>>> position of both atoms, and put the bond along the X direction.
>>>>
>>>>   &CONSTRAINT
>>>>     &FIXED_ATOMS
>>>>       COMPONENTS_TO_FIX      YZ
>>>>       LIST                   1 2
>>>>     &END FIXED_ATOMS
>>>>   &END CONSTRAINT
>>>>
>>>> I think with it, I force the MD to evolve only over the X axis... but
>>>> the initial step gives me a NaN temperature and kinetic energy, no
>>>> matter what initial temperature I use. I attach both files.
>>>>
>>>> The output file prints:
>>>>  Calculation of degrees of freedom
>>>>                                                       Number of atoms:
>>>>         2
>>>>                                  Number of Intramolecular constraints:
>>>>         4
>>>>                                  Number of Intermolecular constraints:
>>>>         0
>>>>                                   Invariants(translation + rotations):
>>>>         5
>>>>                                                    Degrees of freedom:
>>>>        -3
>>>>
>>>>
>>>>  Restraints Information
>>>>                                   Number of Intramolecular restraints:
>>>>         0
>>>>                                   Number of Intermolecular restraints:
>>>>         0
>>>>  ************************** Velocities initialization
>>>> **************************
>>>>  Initial Temperature
>>>>     NaN K
>>>>  COM velocity:                       NaN                 NaN
>>>>       NaN
>>>>  COM position:            5.669178398657      5.669178398657
>>>>  5.669178398657
>>>>  Angular velocity:                   NaN                 NaN
>>>>       NaN
>>>>
>>>>  *******************************************************************************
>>>>
>>>> It seems, the degrees of freedom is: 3N - constrains - invariants...
>>>> Is there a way to force the MD only along one axis to avoid rotation?
>>>>
>>>> Regards!
>>>>
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