<html><head><meta http-equiv="content-type" content="text/html; charset=us-ascii"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div>Dear Rubee,</div><div><br></div><div>the absolute isovalue of an STM cube should generally not be compared directly</div><div>between CP2K and Quantum ESPRESSO.</div><div><br></div><div>CP2K writes a three-dimensional, integrated-LDOS-like quantity obtained by</div><div>summing the orbital densities in the selected bias window. Within the</div><div>Tersoff-Hamann approximation this quantity is proportional to a tunneling</div><div>signal, but it is not a universally normalized or calibrated tunneling</div><div>current. The numerical values can therefore differ between programs even when</div><div>the qualitative image is similar.</div><div><br></div><div>There is also a simple visualization issue to check: an isosurface exists only</div><div>when the requested value lies between the minimum and maximum values present</div><div>in the displayed part of the cube. If an isovalue is outside that range, a</div><div>visualization program may show no surface. Please inspect the actual minimum</div><div>and maximum cube values before selecting the contour.</div><div><br></div><div>For a meaningful comparison, please check that both calculations use the same:</div><div><br></div><div>- sample-bias sign and magnitude;</div><div>- Fermi or reference-energy alignment;</div><div>- occupied or unoccupied energy window;</div><div>- spin state, charge, occupations, and smearing;</div><div>- geometry, cell, vacuum width, and lateral periodicity;</div><div>- spatial region and tip height used to extract the image.</div><div><br></div><div>In CP2K, negative BIAS values sum occupied states between EF+BIAS and EF,</div><div>whereas positive values sum unoccupied states between EF and EF+BIAS.</div><div><br></div><div>For positive bias, enough unoccupied orbitals must be available. Set NLUMO in</div><div>the STM print section and check the output for</div><div><br></div><div> Using a total of ... states</div><div><br></div><div>and for warnings that EFermi+bias lies above the highest computed unoccupied</div><div>orbital. Increase NLUMO until the requested window is fully represented and</div><div>the cube is converged.</div><div><br></div><div>There are two additional CP2K-specific limitations:</div><div><br></div><div>1. The current STM implementation is Gamma-point only. If an explicit k-point</div><div> calculation is active, CP2K warns that STM is not implemented and does not</div><div> generate the corresponding STM cube.</div><div><br></div><div>2. For spin-polarized calculations, the alpha- and beta-spin contributions are</div><div> added to the same cube. CP2K does not currently produce separate</div><div> spin-resolved STM cubes.</div><div><br></div><div>Because CP2K uses atom-centered Gaussian basis functions, the basis-set tails</div><div>in the vacuum region can also affect the decay of the signal. The basis set,</div><div>plane-wave cutoff, relative cutoff, vacuum width, and grid stride should</div><div>therefore be converged for the intended tip height.</div><div><br></div><div>The most robust comparison is a constant-height map at the same physical</div><div>height or a consistently defined constant-current surface. Matching the raw</div><div>numerical isovalue between different electronic-structure programs is usually</div><div>not meaningful.</div><div><br></div><div>If the problem remains, please provide the CP2K input, the relevant STM section</div><div>of the output, the cube header, and the minimum and maximum cube values.</div><div><br></div><div>I have opened a documentation pull request covering these points:</div><div> https://github.com/cp2k/cp2k/pull/5690</div><div><br></div><div>It explains the STM bias windows, occupations, NLUMO and REF_ENERGY, provides</div><div>a minimal input example, describes constant-height and constant-current</div><div>post-processing, and documents the Gamma-point-only and spin-summed</div><div>limitations.</div><div><br></div><div>Best regards,</div><div>Thomas</div><div><br><blockquote type="cite"><div>Am 04.09.2025 um 16:04 schrieb Rubee Soni <rubee1702@gmail.com>:</div><br class="Apple-interchange-newline"><div><div>Dear users,</div><div><br></div>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.<div><br></div><div>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?</div><div><br></div><div>Thank you</div><div><br></div><div>Best Regards,</div><div>Rubee</div><div><br class="webkit-block-placeholder"></div> -- <br> You received this message because you are subscribed to the Google Groups "cp2k" group.<br> To unsubscribe from this group and stop receiving emails from it, send an email to <a href="mailto:cp2k+unsubscribe@googlegroups.com">cp2k+unsubscribe@googlegroups.com</a>.<br> To view this discussion visit <a href="https://groups.google.com/d/msgid/cp2k/6e830ca5-612f-4e71-ad80-755c5f439575n%40googlegroups.com?utm_medium=email&utm_source=footer">https://groups.google.com/d/msgid/cp2k/6e830ca5-612f-4e71-ad80-755c5f439575n%40googlegroups.com</a>.<br> </div></blockquote></div><br></body></html> <p></p> -- <br /> You received this message because you are subscribed to the Google Groups "cp2k" group.<br /> To unsubscribe from this group and stop receiving emails from it, send an email to <a href="mailto:cp2k+unsubscribe@googlegroups.com">cp2k+unsubscribe@googlegroups.com</a>.<br /> To view this discussion visit <a href="https://groups.google.com/d/msgid/cp2k/EC3BAC66-0EAD-4237-94FD-08EA934EB29C%40gmail.com?utm_medium=email&utm_source=footer">https://groups.google.com/d/msgid/cp2k/EC3BAC66-0EAD-4237-94FD-08EA934EB29C%40gmail.com</a>.<br />