[CP2K-user] [CP2K:21461] Regarding the problem of using three functionals to optimize the spatial structure of the high spin state of the spin crossover material [Fe(pz)][Ni(CN)4]

Daniel Lee danielleelore at gmail.com
Tue May 13 02:14:44 UTC 2025


Hi, greetings:

I hope to use CP2K to optimize the spatial structure of the high spin state 
of the spin crossover periodic crystal [Fe(pyrazine)][Ni(CN)4] (a MOF type 
material, the crystal structure is shown in Figure 1 
<https://imgur.com/a/2xhQjKi>).

This material is in a low spin state at normal pressure and low 
temperature, which is a diamagnetic state. The d-layer electronic 
configuration of Fe(ii) is t2g6, spin is 0, and Ni spin is 0. It will 
automatically transform into a high spin state at normal pressure and 300K, 
which is a paramagnetic state, the d-layer electronic configuration of 
Fe(ii) becomes t2g4eg2, spin is 2, and Ni spin is 0; the difference between 
the high and low spin states in the spatial structure is mainly reflected 
in the different bond lengths of the Fe-N octahedron (high---2.1+A, 
low---1.9+A), and the bond lengths of other chemical bonds remain basically 
unchanged. The unit cell of this material has 20 atoms.

There are several studies in the field of spin crossover research (such as 
a recent one DOI: 10.1063/5.0157187) showing that tpssh, b3lyp and m06l are 
three functionals that are fairly good at describing the properties of spin 
crossover materials, so I would like to use these three functionals to 
optimize the spatial structure of the high and low spin states of 
[Fe(pyrazine)][Ni(CN)4] and compare the simulation results of different 
functionals.

Recently, a study (10.1021/acs.jpcc.2c01030) used quantum espresso to 
simulate the PBE+D2 calculation of a similar material 
[Fe(pyrazine)][Pt(CN)4]. Therefore, I used the method of this study to 
optimize the spatial structure of the high and low spin states of 
[Fe(pyrazine)][Ni(CN)4] using QE's PBE+D2. After setting different spin 
polarization parameters for the input files corresponding to the high and 
low spin state according to the experimental data (see attachment 1 for the 
high spin input file and attachment 2 for the low spin input file), the 
vc-relax calculations converged successfully, and the results also 
reflected the spatial structure and magnetic properties of the high and low 
spin states. The calculated bond length data was in line with expectations 
(see Figure 2 <https://imgur.com/9hMUtYb> for the optimized unit cell 
structure for high spin state and Figure 3 <https://imgur.com/feFHAYl> for 
low spin state).

Then I tried to use tpssh, b3lyp and m06l functionals in QE to optimize 
these spatial structures. However, for tpssh and m06l functionals, I did 
not find a pseudopotential group that could make the optimization run 
normally without errors (for example, the most common error is "Error in 
routine cdiaghg (161): S matrix not positive definite"). On the other hand, 
 although the SCF converged, the b3lyp had the problem that the calculation 
of "Using ACE for calculation of exact exchange" would keep looping for a 
long time on a hpc cluster(see attachment 3). Therefore, I decided to try 
to use CP2K to simulate the three functionals of tpssh, b3lyp and m06l.

I used Multiwfn to generate input files for high and low spin state spatial 
structure optimization. The low spin state was successfully simulated using 
three functionals: tpssh (expanding the cell to 2*2*2 and only counting the 
Γ point, attachment 4), b3lyp (expanding the cell to 2*2*2 and only 
counting the Γ point), and m06l (using the primitive cell, with the K point 
of 6*6*6, attachment 5). The optimization results of the two hybrid 
functionals t and b were in line with expectations (see Figures 4 
<https://imgur.com/H0NeEv1> and 5 <https://imgur.com/oYAENpH>, 
respectively). Although the Fe-N bond length calculated by m was in the low 
spin state range, the bond lengths that should have been equal were 
different (see Figure 6 <https://imgur.com/Jo2x2a9>).

In the high spin state simulation, although b3lyp (attachment 6) was able 
to converge and the program successfully exited, the optimized structure 
obtained was not ideal. Some of the 8 primitive cells in the supercell were 
in the high spin state (Fe-N bond length was greater than 2), and some were 
in the low spin state (Fe-N bond length was less than 2), and the Fe-N bond 
lengths in the same primitive cell were not all equal (see Figure 7 
<https://imgur.com/xm2jKsp>). When I used tpssh to optimize, the settings I 
used were almost the same as those of b3lyp. However, scf did not converge 
(it oscillated around 10E-5 in the output file), so I increased EPS_SCF 
from 10E-6 to 10E-5 (Appendix 7) and tried the calculation again. This 
time, SCF converged, but the forces in the geometry optimization (Maximum 
gradient data in the output file) could not converge to the default 4.5E-4 
(the closest it could get was 8E-4) after running for more than 12 hours. 
After checking the pdb file of the output geometry structure change data, I 
found that the overall structure only had obvious changes in the initial 
optimization stage, and no longer changed in the later stage. In addition, 
the later structure had similar problems as the b3lyp optimization result 
(see Figure 8 <https://imgur.com/rci6EyL>). When optimizing with m06l 
(input attachment 8), the SCF generally oscillates between 10-2 and 10-3 
(output attachment 9). I tried some combinations of basis vectors and 
pseudopotentials (https://cp2k-basis.pierrebeaujean.net/), such as the 
basis vector and pseudopotentials for PBE and MGGA, but the SCF convergence 
problem was not solved.

I also tried to use CP2K's PBE+D3 method (using primitive cells and 6*6*6 K 
points) to optimize the high-spin state (attachment 10). After running for 
more than 10 hours, the situation similar to tpssh high-spin state 
simulation in which the force cannot reach the convergence limit appeared. 
The output geometry structure change trajectory from pdb file is almost 
stable in the later stage and the bond length and other data are consistent 
with the high-spin state (see Figure 9 <https://imgur.com/n99RAWL>). In 
addition, the high-spin state optimization results of PBE with 2*2*2 
expanded cells as the initial guess of tpssh optimization also produced the 
similar result that the force cannot reach the convergence limit in the 
later stage and the optimized structure in the later stage has similar 
problems as described above (see Figure 10 <https://imgur.com/Y6pIhtn>).

Then I tried to use CDFT to fix the magnetic moment of Fe, but the two 
functionals pbe and tpssh (see attachment 11) I tried faced a problem that 
the "Target value of constraint" and "Current value of constraint" 
polarized after the program ran for a certain period of time (see 
attachment 12).

Dear CP2K users, are there any problems in my various input file parameter 
settings shown above? Is there any way to make the conventional DFT or CDFT 
structure optimization results of tpssh, m06l or b3lyp successfully 
converge to a satisfactory high spin state?

Thank you very much, your help are much appreciated.

                                                                                                                       
Daniel Lee

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!    total energy              =    -794.17980073 Ry
     estimated scf accuracy    <          9.0E-11 Ry
     smearing contrib. (-TS)   =       0.00000000 Ry
     internal energy E=F+TS    =    -794.17980073 Ry

     total magnetization       =    -0.00 Bohr mag/cell
     absolute magnetization    =     0.14 Bohr mag/cell

     convergence has been achieved in   5 iterations

     Using ACE for calculation of exact exchange
     ACE projected onto    60 (nbndproj) and applied to    60 (nbnd) bands

     total energy              =    -794.17980532 Ry
     Harris-Foulkes estimate   =    -794.17980532 Ry
     est. exchange err (dexx)  =       0.00000458 Ry
     - averaged Fock potential =      43.54602550 Ry
     + Fock energy (ACE)       =     -21.77302900 Ry

     EXX: now go back to refine exchange calculation

     total cpu time spent up to now is   595267.8 secs

     per-process dynamical memory:   528.6 Mb

     Self-consistent Calculation

     iteration #  1     ecut=   100.00 Ry     beta= 0.50
     Davidson diagonalization with overlap
     ethr =  1.83E-13,  avg # of iterations =  9.6

     total cpu time spent up to now is   595304.6 secs

     total energy              =    -794.17980945 Ry
     estimated scf accuracy    <       0.00000113 Ry

     total magnetization       =    -0.00 Bohr mag/cell
     absolute magnetization    =     0.14 Bohr mag/cell

     iteration #  2     ecut=   100.00 Ry     beta= 0.50
     Davidson diagonalization with overlap
     ethr =  1.13E-09,  avg # of iterations =  2.0

     total cpu time spent up to now is   595316.9 secs


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