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.