1.4. 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)

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):

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):

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:

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.

copy orca.out temp.out
edit temp.out

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

1.4.1. 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:

! 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:

! 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 General Structure of the Input File and the section Hello water! Your first ORCA calculation will guide you through your first steps with ORCA.