Input File¶
This page walks through the Gaussian input file used for the NMR calculation on the formic acid dimer.
Complete Input¶
%nprocshared=1
%mem=2GB
%chk=formicacid-opt-nmr.chk
# nmr=giao 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
Route Section¶
| Keyword | Meaning |
|---|---|
NMR=GIAO |
Requests an NMR shielding tensor calculation using Gauge-Including Atomic Orbitals. |
B3LYP/6-31G |
Method and basis set — the same combination used consistently across this site's Gaussian tutorials. |
geom=connectivity |
Reads the explicit bonding pattern given after the coordinates, rather than inferring it from interatomic distances. |
Note that there is no opt keyword here — this run is a single-point NMR calculation. The geometry below is assumed to already be a converged minimum (in this case, the formic acid dimer geometry used elsewhere on this site), not something this job will optimize.
Molecule Specification¶
The molecule is the formic acid dimer — two formic acid monomers linked by a pair of hydrogen bonds, arranged with a center of symmetry. The connectivity block reflects this: atoms 1–5 form one monomer and atoms 6–10 the other, each internally bonded the same way (1.0/1.5/2.0 bond orders reflecting the partial double-bond character of the carboxylic acid group).
This symmetry is not just structural — it shows up directly in the output (see Output File), since the two monomers are related by the molecule's symmetry and should give matching shielding values atom-for-atom.
What's different from an Opt+Freq input¶
If you've worked through Optimization + Frequency already, the only structural difference here is the route line: opt freq is replaced with nmr=giao, and the geometry provided is treated as fixed rather than as a starting guess to be optimized.