[QE-users] Relaxation of MoN/MoP2 heterostructure stops before force convergence despite SCF convergence

[Giovanni Cantele]

Hi,

generally speaking it would have been better to provide the input and
output files, the more details are given, the more
complete the answer can be.

If you're carrying out a geometry optimization it is because you want to
obtain reasonable bond lengths, lattice plane or
interlayer distances, and so on. Getting the last but not converged
geometry might be reasonable when the code gets stuck
in the optimization loop providing small forces. The total force you obtain
seems large. So i would definitely increase nstep,
set calculation = 'restart' and let pw.x complete the bfgs loop.

A relevant parameter would be also how energy is changing across the bfgs
steps.

Giovanni
-- 

Prof. Giovanni Cantele
Dipartimento di Fisica "Ettore Pancini"
Universita' degli Studi di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at unina.it
Phone: +39 081 676910

Web page: https://sites.google.com/view/giovanni-cantele/home


Il giorno sab 10 gen 2026 alle ore 13:33 BINOY CHANDRA DEY via users <
users at lists.quantum-espresso.org> ha scritto:

> Dear Quantum ESPRESSO Community,
>
> I am currently performing a geometry optimization of a MoN/MoP₂
> heterostructure and would appreciate your guidance regarding the
> convergence behavior I am observing.
>
> *System construction:*
> The heterostructure was built by cutting bulk MoN and MoP₂ into slabs
> using experimentally suggested Miller indices. The two slabs were stacked
> vertically along the z direction, with ~20 Å of vacuum added to avoid
> spurious interactions. The initial interlayer distance between the slabs
> was set to ~5 Å. All atoms were allowed to relax.
>
> *Calculation details:*
>
>    -
>
>    calculation = 'relax'
>    -
>
>    Ionic dynamics: BFGS
>    -
>
>    Dispersion correction: DFT-D3
>    -
>
>    Fixed cell (no vc-relax)
>    -
>
>    nstep = 50
>
> *Problem description:*
> Although the SCF convergence is achieved smoothly at every ionic step
> (estimated SCF accuracy ~10⁻⁸ Ry), the geometry optimization does not
> converge within the specified number of BFGS steps. The run stops after
> reaching nstep = 50, without printing final relaxed coordinates.
>
> At the last few ionic steps, the total energy continues to decrease
> monotonically; however, the total force remains relatively large (~0.063
> Ry/Bohr), indicating that the structure is still far from force convergence.
>
> Below I include excerpts from the last iterations for reference.
> ------------------------------
>
> *Penultimate BFGS step:*
>
> total energy              =  -3822.65535130 Ry
> estimated scf accuracy    <      0.00000009 Ry
>
> convergence has been achieved in 32 iterations
> number of scf cycles      = 49
> number of bfgs steps      = 48
>
> energy old = -3822.65309557 Ry
> energy new = -3822.65535130 Ry
>
> CASE: energy_new < energy_old
>
> new trust radius          = 0.5692417702 bohr
> new conv_thr              = 0.0000001320 Ry
>
> Total force = 0.063444
>
> *Final BFGS step (terminated due to nstep limit):*
>
> total energy              =  -3822.65857840 Ry
> estimated scf accuracy    <      0.00000010 Ry
>
> convergence has been achieved in 40 iterations
> number of scf cycles      = 50
> number of bfgs steps      = 49
>
> energy old = -3822.65535130 Ry
> energy new = -3822.65857840 Ry
>
> CASE: energy_new < energy_old
>
> new trust radius          = 0.4868398914 bohr
> new conv_thr              = 0.0000002314 Ry
>
> Total force = 0.063444
>
> The stress tensor is also printed, but since the calculation is a slab
> geometry with large vacuum and a fixed cell, I have not interpreted the
> stress as a convergence criterion.
>
> *My questions are:*
>
>    1.
>
>    Is it physically reasonable to use the final BFGS geometry as the
>    relaxed structure, even though the force convergence threshold has not been
>    met?
>    2.
>
>    Would simply increasing nstep and restarting the relaxation from the
>    last configuration be sufficient, or is it strongly recommended to use
>    selective dynamics (e.g., freezing bottom layers of the slabs) for such
>    heterostructure systems?
>    3.
>
>    Are there any best-practice strategies or parameter adjustments (e.g.,
>    force thresholds, trust radius, relaxation protocol) that you would
>    recommend for improving convergence in slab-based heterostructure
>    relaxations with large vacuum?
>
> Best regards,
> Binoy Chandra Dey
> Ph.D. Scholar
> IIT Bhubaneswar
>
> ------------------------------
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