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Preparing the Gaussian Input File

The example below tests the stability of the RB3LYP/6-31G wavefunction of a hydrogen-bonded formic acid dimer.

%nprocshared=1
%mem=2GB
%chk=formicacid-stability.chk
# stable b3lyp/6-31g geom=connectivity

Title Card Required

0 1
 C                 -0.11131000    1.89246900   -0.00024600
 H                 -0.16211200    2.98106100   -0.00117300
 O                  1.14244500    1.44697500    0.00021700
 H                  1.22534700    0.42975100    0.00008700
 O                 -1.14244500    1.18616900    0.00010300
 C                  0.11131000   -1.89246900   -0.00024600
 H                  0.16211200   -2.98106100   -0.00117300
 O                 -1.14244500   -1.44697500    0.00021700
 O                  1.14244500   -1.18616900    0.00010300
 H                 -1.22534700   -0.42975100    0.00008700

 1 2 1.0 3 1.5 5 2.0
 2
 3
 4
 5
 6 7 1.0 8 1.5 9 2.0
 7
 8
 9
 10

Link 0 Commands

  • %nprocshared=1 — number of shared-memory processors requested.
  • %mem=2GB — memory allocation for the job.
  • %chk=formicacid-stability.chk — checkpoint file storing the converged orbitals; useful for restarting or for feeding a follow-up job.

The Route Section

# stable b3lyp/6-31g geom=connectivity
  • stable — requests a wavefunction stability analysis. Used alone, it is equivalent to Stable=Opt: Gaussian first converges the SCF/DFT wavefunction, then tests it for instabilities, and — if an instability is found — automatically rotates the orbitals along the softest mode and re-optimizes, repeating until a stable solution is reached.
  • b3lyp/6-31g — the level of theory whose wavefunction is being tested; any HF or DFT method can be used here.
  • geom=connectivity — tells Gaussian to read an explicit bonding table after the coordinates rather than inferring connectivity from distances. It has no effect on the electronic structure calculation itself.

Other Stability Options

stable accepts several keywords that narrow or broaden which instability channels are tested:

  • Stable=Real — test only real (non-complex) instabilities.
  • Stable=Triplet — test only the spin-symmetry-breaking (RHF→UHF) channel.
  • Stable=Complex — also allow complex-valued orbital rotations.
  • Stable=CalcAll or Stable=ReadFC — control how much of the underlying Hessian information is (re)computed versus read from a previous step.
  • Stable=Opt (the default when only stable is given) — automatically re-optimize the SCF if an instability is found.

Used with no options, as in this example, Gaussian tests the combined real internal channel — covering both RHF→RHF and RHF→UHF instabilities in a single diagonalization.

Molecule Specification

0 1 sets the charge (0) and spin multiplicity (1, closed-shell singlet). The ten atoms describe two formic acid molecules arranged as a cyclic, doubly hydrogen-bonded dimer — a system whose delocalized, resonance-averaged O–C–O bonding makes it a reasonable candidate to check for a hidden lower-energy orbital arrangement.

The connectivity block that follows the coordinates (1 2 1.0 3 1.5 5 2.0 …) lists bonded atom pairs and formal bond orders for each formic acid unit; it is bookkeeping for geom=connectivity and does not influence the SCF result.

A Note on Restarts

If a stability run reports an instability and you want to inspect the improved wavefunction separately, the checkpoint file lets you restart with Guess=Read and continue from the rotated orbitals rather than repeating the whole calculation.