[QE-users] How H2O molecule reflect DFT scf calculation at 0 Kelvin calculation

[Jayraj Anadani]

Thank you sir, for your reply
but I have one particular molecular system of 10-20 atoms which I am
getting from a simulation of a large atom (around 10000 atoms) at a 300K
temperature from a molecular dynamic simulation (LAMMPS). and I also did
optimization of that MD structure which is amorphous metallic glass. Now,
 Can I do dft study of a small part of a single molecule of a large MD
simulation(~10000 atoms) ?
till now i did below things but i am bit confused which way is best way to
study DFT . i also dis AIMD/CPMD but for metallic glass CPMD doesn't work
good with PAW PP.
so,
1. Can I do direct SCF calculation(without relax) by giving a condition of
*forc_conv_thr* and *tot_conv_thr* with creating a vacuum(for a
non-periodic system) of around 15 Angstrom to converge properly. *OR*
2. I should do the RELAX calculation with vacuum and then go to the
*scf* calculation
to find the total energy and then *nscf*.

***questions:
1. what if I relax a system and position is changed a bit(which is obvious)
to minimize the total energy but that energy is at zero kelvin dft
calculation and I want to study the system at 300K. So how can small parts
of a structure reflect large MD simulation structural properties?

Thank you
regards


jayraj anadani
Ph.D. Research scholar Department of physics
sardar patel university
india

On Fri, Aug 26, 2022 at 5:00 PM Stefano de Gironcoli <degironc at sissa.it>
wrote:

> Dear JAYRAJ ANADANI,
>
>   calculation="scf"  corresponds to a minimization of the total energy
> at fixed atomic positions. you chose where to put the ions. the code
> does its best to return the corresponding energy.
>
>   calculaiton="relax" corresponds to a series of scf calculations,
> followed by forces evaluation and geometry update to minimize the energy
> of the system. The code does its best to converge to a minimum (non
> necessarily the global minimum) in the GS potential energy surface.
>
>   there is no temperature in any of these calculations.
>
>   if you want to simulate the system at finite (classical) temperature
> for the atoms you should perform MD simlations
>
> stefano
>
> On 26/08/22 05:02, Jayraj Anadani wrote:
> > Dear QE
> >               I put H2O molecular atoms small far apart from each
> > other within large supercell so that the periodic image of the H2O do
> > not interact with each other and run a scf calculation my convergence
> > done successfully with good amount of accuracy upto 10^-9. But it is
> > not reflecting original H2O molecule at 300 Kelvin...but when i
> > relaxing these atoms and then did scf calculation they are reflecting
> > H2O at 300k by observing bond angel and length.. how ? And my question is
> > 1) scf is also doing calculation using iteration method algorithm by
> > using electronic densities at ground state so why they converge when
> > two hydrogen atoms and one oxygen atom are not at appropriate
> > distance.still scf converge how ?
> > 2) also by doing relax calculation all of sudden two hydrogen and one
> > oxygen come closer and they formed a bond and bond angel which is
> > reflect 300k H2O molecule structure. So how two hydrogen and one
> > oxygen atoms knows during relaxation that i have to make structure at
> > a room temperature?
> >
> >
> > Thank you
> > Regards
> >
> > JAYRAJ ANADANI
> > Ph.D Research scholar
> > Department of physics
> > Sardar Patel University
> > Anand,Gujarat,
> > India
> >
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