SIGSEGV in Klopman-Ohno Coulomb gradient (klopmanohno.f90) for large systems (>400 atoms) after successful SCF convergence

[baixingle]

Description

xTB 6.7.1 (edcfbbe) consistently crashes with SIGSEGV during gradient computation for large systems (~426 atoms), immediately after successful SCF convergence. The crash occurs in the Klopman-Ohno Coulomb integral code (klopmanohno.f90), regardless of method (GFN1/GFN2), solvent model, or parallelization settings.

Environment

• xTB version: 6.7.1 (edcfbbe), compiled 2024-07-22 with ifort
• OS: Linux (CentOS 7, x86_64)
• Installation: pre-compiled binary from GitHub releases

Steps to Reproduce

1. Prepare a 426-atom system (Bi-water interface, 64 Bi + 362 water molecules)
2. Run a single-point or optimization calculation:

xtb input.xyz --gfn1 --parallel 1 2>&1

SCF converges normally after ~60-80 iterations
4. SIGSEGV occurs during gradient computation
Test Matrix
Config
SCF
Result
GFN1, no solvent, serial
✅ converged in 80 iters
SIGSEGV
GFN1 + ALPB water, serial
✅ converged in 61 iters
SIGSEGV
GFN1 + ALPB water, 64 threads
✅ converged
SIGSEGV
GFN2, no solvent, serial
✅ converged
SIGSEGV
GFN2 + ALPB water, 64 threads
❌ crashes during init
SIGSEGV (init)
Same input, 330-atom water box
✅
✅ No crash
Stack Trace
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forrtl: severe (174): SIGSEGV, segmentation fault occurred
Image              PC                Routine            Line        Source
xtb                000000000046E88A  Unknown               Unknown  Unknown
xtb                0000000002D1BD40  Unknown               Unknown  Unknown
xtb                0000000000E16DFD  xtb_coulomb_klopm         508  klopmanohno.f90
xtb                0000000002C45313  Unknown               Unknown  Unknown
xtb                0000000002BFEF40  Unknown               Unknown  Unknown
xtb                0000000002BCBB08  Unknown               Unknown  Unknown
xtb                0000000000E16B63  xtb_coulomb_klopm         508  klopmanohno.f90
xtb                0000000000E13B64  xtb_coulomb_klopm         460  klopmanohno.f90
xtb                0000000000C2F0DA  xtb_scf_mp_scf_.A         798  scf_module.F90
xtb                00000000008FFEAF  xtb_xtb_calculato         258  calculator.f90
xtb                0000000000438C70  xtb_prog_main_mp_         687  main.F90
xtb                0000000000450EA2  MAIN__                     57  primary.f90
Key Observations
• SCF always converges — the crash only happens when computing gradients
• Crash point: klopmanohno.f90 lines 460 and 508 (Klopman-Ohno Coulomb integral derivatives)
• Not caused by: solvent model (happens without GBSA/ALPB), OpenMP threading (happens serial), stack size (ulimit -s unlimited doesn't help), UHF or electronic temperature
• System size threshold: 330-atom system works fine; 426-atom system triggers the bug
• GFN-FF works perfectly on the same system (different code path, no Klopman-Ohno integrals)
• SCF output is normal with reasonable energies (~ -849 Eh for GFN1, ~ -849 Eh with ALPB)
Suspected Cause
Array dimension mismatch in klopmanohno.f90 — likely a fixed-size allocatable or stack array that overflows when the number of atom pairs exceeds a threshold around 400 atoms during gradient evaluation. The integral evaluation during SCF uses a different code path that doesn't hit the same limit.
Workaround
Use GFN-FF for large systems (> ~350 atoms), or reduce system size below the threshold for GFN1/GFN2 calculations.

[chselz]

Hi, thank you for your interest in xTB and for reporting this. As a lot of bugs have been fixed since the latest release, i would try again using the latest bleeding edge version of xTB. Further it might be a good idea to also use the --tblite flag to use the tblite as a backend for the GFN Hamiltionians. tblite provides a much cleaner implementation than the native one and very likely does not have this bug.

You can also try to test the xTB binary with g-xTB support (--gxtb) which will give you much better results than gfn1 or gfn2.
You can find the precompiled binaries here: https://github.com/grimme-lab/g-xtb/releases/tag/v2.0.1