Gaussian input files have the file extension .gjf or .com. Jobs will read in data from a checkpoint (.chk) file, or write to new file. Logfiles (.log) contain all the information about the job, and whether it failed or ran successfully.

Water Example

Here is what a basic input for water would look like.

%mem=32MB
% chk=water.chk
# opt freq geom b3lyp/6-31+g(d,p) geom=connectivity integral=ultrafine scf=maxcyc=1024

water

0 1
O 1.80172602 0.06746038 0.00000000
H 2.76172599 0.06770148 0.00000000
H 1.48149872 0.97247667 0.00000000

This input, with explanatory comments marked with ### is:

%mem=32MB ### system resources (both % words or %words OK)
% chk=water.chk ### specifies the checkpoint file to write
# opt freq geom b3lyp/6-31+g(d,p) geom=connectivity integral=ultrafine scf=maxcyc=1024 ### Job specifics–NO ENTER KEY
### I am a required blank line
water ### Titlecard: this line is required
### I am a required blank line
0 1 ### Charge, spin multiplicity
O 1.80172602 0.06746038 0.00000000
H 2.76172599 0.06770148 0.00000000
H 1.48149872 0.97247667 0.00000000
### I am a required blank line

Calculate → Gaussian Calculation Set-Up

The calculation setup window, with the job type dropdown shown. The options are Energy, Optimization, Frequency, Opt+Freq, IRC, Scan, Stability, and NMR.
Different Gaussian job types.

Job Type

  1. Energy: Calculates the energy and wavefunction at a single, fixed geometry
  2. Optimization: Attempts to find the structure’s minimum energy configuration
  3. Frequency: Gives thermochemical properties of the structure
  4. Opt + Freq: Performs both optimization and frequency
  5. IRC: Asks to follow a reaction path by integrating the intrinsic reaction coordinate
  6. Scan: Scans the potential energy surface by performing single-point energy calculations
  7. Stability: Checks stability by determining if imaginary (negative) frequencies exist (Yes? That’s bad, unless a transition state.)
  8. NMR: Predicts NMR shielding tensors

Method

  • Method: the “school of thought” for the wavefunction
  • Basis set: contains all the functions that represent the wavefunction
    • Higher-order basis sets are often more accurate, but at a higher computational/time cost
    • Diffuse functions (ex: +, ++) describe very electronegative atoms
    • Polarization functions (ex: d, p) allow for angular flexibility (i.e., reduces the strain from lots of electrons)
  • Charge: overall charge of the structure
  • Spin: pairing of the electrons
    • Singlet: All electrons paired
    • Multiplicity = Number of unpaired electrons + 1
calculation setup window, with the method tab selected. The first line is method with Ground State, Hartree--Fock, and Default Spin selected. The next line is Basis Set with 3-21G entered in the first box and the next two empty. The third line has Charge with 0 and Spin as Singlet.
The method tab.

Solvation

  • Implicit: Solvent is a polarizable continuum with dielectric constant, ε
    • Not terribly costly
    • Cannot model specific interactions, like hydrogen bonds
    • Magician waving a wand  there’s some magic happening, but you can’t see it
  • Explicit: Want a solvent? Build it in. Put it there.
    • Expensive…
    • Can get stuff like hydrogen bonds
    • As magician’s apprentice, you see all the things going into the “magic”
The left panel is a methane molecule, CH4, on a white background. The caption is Vacuum (Gas Phase). The middle panel is methane surrounded by a slight white gap before a red and mark hashed pattern. The caption is implicit solvent. The right panel is methane with about 4 waters, H2O, drawn. The caption is explicit solvent.
Different solvation models.