[CP2K-user] [CP2K:22376] STM simulation shows image only at higher isovalue

[Thomas Kühne]

Dear Rubee,

the absolute isovalue of an STM cube should generally not be compared directly
between CP2K and Quantum ESPRESSO.

CP2K writes a three-dimensional, integrated-LDOS-like quantity obtained by
summing the orbital densities in the selected bias window. Within the
Tersoff-Hamann approximation this quantity is proportional to a tunneling
signal, but it is not a universally normalized or calibrated tunneling
current. The numerical values can therefore differ between programs even when
the qualitative image is similar.

There is also a simple visualization issue to check: an isosurface exists only
when the requested value lies between the minimum and maximum values present
in the displayed part of the cube. If an isovalue is outside that range, a
visualization program may show no surface. Please inspect the actual minimum
and maximum cube values before selecting the contour.

For a meaningful comparison, please check that both calculations use the same:

- sample-bias sign and magnitude;
- Fermi or reference-energy alignment;
- occupied or unoccupied energy window;
- spin state, charge, occupations, and smearing;
- geometry, cell, vacuum width, and lateral periodicity;
- spatial region and tip height used to extract the image.

In CP2K, negative BIAS values sum occupied states between EF+BIAS and EF,
whereas positive values sum unoccupied states between EF and EF+BIAS.

For positive bias, enough unoccupied orbitals must be available. Set NLUMO in
the STM print section and check the output for

  Using a total of ... states

and for warnings that EFermi+bias lies above the highest computed unoccupied
orbital. Increase NLUMO until the requested window is fully represented and
the cube is converged.

There are two additional CP2K-specific limitations:

1. The current STM implementation is Gamma-point only. If an explicit k-point
   calculation is active, CP2K warns that STM is not implemented and does not
   generate the corresponding STM cube.

2. For spin-polarized calculations, the alpha- and beta-spin contributions are
   added to the same cube. CP2K does not currently produce separate
   spin-resolved STM cubes.

Because CP2K uses atom-centered Gaussian basis functions, the basis-set tails
in the vacuum region can also affect the decay of the signal. The basis set,
plane-wave cutoff, relative cutoff, vacuum width, and grid stride should
therefore be converged for the intended tip height.

The most robust comparison is a constant-height map at the same physical
height or a consistently defined constant-current surface. Matching the raw
numerical isovalue between different electronic-structure programs is usually
not meaningful.

If the problem remains, please provide the CP2K input, the relevant STM section
of the output, the cube header, and the minimum and maximum cube values.

I have opened a documentation pull request covering these points:
  https://github.com/cp2k/cp2k/pull/5690

It explains the STM bias windows, occupations, NLUMO and REF_ENERGY, provides
a minimal input example, describes constant-height and constant-current
post-processing, and documents the Gamma-point-only and spin-summed
limitations.

Best regards,
Thomas

> Am 04.09.2025 um 16:04 schrieb Rubee Soni <rubee1702 at gmail.com>:
> 
> Dear users,
> 
> I am a beginner CP2K user and I am trying to perform STM image simulations for a molecule adsorbed on a metal surface. I used the geometry optimized with Quantum ESPRESSO.  and I have prepared my input by taking reference from the STM tutorial provided on the CP2K website.
> 
> The issue is that using CP2K, the STM image only appears at relatively high isovalue (around 1E-03), while at lower isovalues (e.g., 1E-05) no image is visible. Quantum ESPRESSO produces an STM image even at much lower isovalue (around 1E-06). What could be the reason for this difference?
> 
> Thank you
> 
> Best Regards,
> Rubee
> 
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