# Anatomy of an input file¶

Below is an example of an input file from the HF tutorial with some extra comments.

system
name: H2O
charge: 0
end system

!A comment
do
ground state
end do

!Another comment
method
hf
!A third comment
end method

geometry
basis: cc-pVDZ
H          0.86681        0.60144        0.00000
H         -0.86681        0.60144        0.00000
O          0.00000       -0.07579        0.00000
end geometry


The input consists of a set of sections. A section begins with the name of the section and ends with an end statement.

section name
...
end section name


In the example input above, the system, do, method, and geometry sections are given. These four sections are needed in every input.

The program will do a sanity check of the input, and throw an error if it detects a nonexisting section. See here for a list of all sections and keywords.

Keywords are placed in the sections:

section name
keyword 1: value
keyword 2
end section name


Many keywords have default values and are not required unless you don’t want the default. A few keywords are always required. In general, the set of required keywords and sections will depend on the type of calculation.

Some keywords require a value. These keywords are followed by : and then the value:

keyword 1: value


Other keywords do not require a value, e.g. ground state in the example above.

The input file is not case sensitive. Whitespaces can be added, for instance to add indentation to your input file. Lines can be commented out using ! (except in the geometry section). Blank lines between sections are ignored.

The geometry section must be the final section in the input.