```{index} Running ORCA, starting ORCA
```

(sec:running)=
# Running ORCA

Under Windows the program is called from the command prompt! (Make sure
that the PATH variable is set such that the orca executables are
visible)

```sh
orca orca.inp > orca.out
```

Under UNIX based operating systems the following call is convenient
(here also: make sure that the PATH variable is set to the directory
where the orca executables reside):

```sh
orca orca.inp >& orca.out &
```

The program writes to *stdout* and *stderr*. Therefore the output must
be redirected to the file *orca.out* in this example. *orca.inp* is a
free format ASCII file that contains the input description. The program
will produce a number of files *orca.x.tmp* and the file *orca.gbw*.
The "\*.gbw" file contains a binary summary of the calculation. GBW
stands for "Geometry-Basis-Wavefunction". Basically this together with
the calculation flags is what is stored in this file. You need this file
for restarting SCF calculations or starting other calculations with the
orbitals from this calculation as input. The "\*.tmp" files are
temporary files that contain integrals, Fock matrices etc. that are used
as intermediates in the calculation. If the program exits normally all
of these files are deleted. If it happens to crash you have to remove
the files manually (`rm orca*.tmp` under Unix or `del orca*.tmp` under
Windows). In case you want to monitor the output file while it is
written, you can use the command (under Unix):

```sh
tail -f orca.out
```

to follow (option -f) the progress of the calculation. Under Windows you
have to either open another command shell and use:

```bat
type orca.out
type orca.out |more
```

or you have to copy the output file to another file and then use any
text editor to look at it.

```bat
copy orca.out temp.out
edit temp.out
```

you cannot use `edit orca.out` because this would result in a sharing
violation.


(sec:exampleinput)=
## A simple example 

A great strength of ORCA is the simple structure of its input file. ORCA input files can be easily generated by any simple text editor.
Simply create a file with an `.inp` extension, e.g. `orca.inp`, and add the input information to it. The most basic input file looks like this:

```orca
! METHOD

*XYZ CHARGE MULTIPLICITY
Atom1   X1  Y1  Y1
Atom2   X2  Y2  Y2
*
```

Here, we define our quantum mechanical method of choice via the so called *simple input line* that starts with an exclamation mark. Afterwards, we define the structure of our molecule, its net charge, and its multiplicity (M = 2S+1). In this example, we chose cartesian coordinates in XYZ format given in Ångström.
The coordinates block must be closed by an `*`.

An example, using Hartee-Fock theory and a def2-SVP basis set for the neutral, singlet dihydrogen molecule looks like this:

```orca
! HF DEF2-SVP

*XYZ 0 1
H   0  0  0
H   0.74  0  0
*
```

More details on the ORCA input file syntax can be found in the section {ref}`sec:essentialelements.input`
and the section {ref}`sec:hellowater` will guide you through your first steps with ORCA.