Input File¶

This section explains the ORCA input file used to perform a Restricted Hartree–Fock (RHF) calculation on a water molecule. Although this is a simple example, it introduces the basic structure common to most ORCA input files. Once these components are understood, the same format can be extended to more advanced calculations such as CASSCF, CASPT2, and NEVPT2.

The complete input file is shown below.

! RHF def2-SVP

%pal
   nprocs 1
end

* xyz 0 1
O   0.0000000000    0.0000000000    0.1068300000
H   0.0000000000    0.7851780000   -0.4273190000
H   0.0000000000   -0.7851780000   -0.4273190000
*

Input File Structure¶

An ORCA input file generally consists of four parts:

Method and basis set specification
Optional calculation settings
Molecular geometry
End of geometry marker

Each of these sections is described below.

Method and Basis Set¶

! RHF def2-SVP

The first line begins with the ! character, which is used in ORCA to specify the computational method and keywords.

In this example:

RHF requests a Restricted Hartree–Fock calculation.
def2-SVP specifies the Ahlrichs split-valence polarized basis set.

Because the water molecule is a closed-shell system, every occupied molecular orbital contains two electrons with opposite spins, making RHF the appropriate Hartree–Fock method.

The def2-SVP basis set provides a good balance between computational efficiency and accuracy, making it suitable for introductory calculations and geometry optimizations.

Parallelization¶

%pal
   nprocs 1
end

The %pal block controls parallel execution.

The keyword

nprocs 1

tells ORCA to use a single processor core.

For larger calculations, this value can be increased to utilize multiple CPU cores. For example,

nprocs 8

would request eight processor cores.

Although this example uses only one core, including the %pal block is good practice because it can easily be modified for larger systems.

Molecular Geometry¶

* xyz 0 1

This line indicates that Cartesian coordinates will follow.

The values have the following meaning:

Entry	Meaning
`xyz`	Cartesian coordinates
`0`	Total molecular charge
`1`	Spin multiplicity (singlet)

Since water is electrically neutral and all electrons are paired, its charge is 0 and its multiplicity is 1.

Atomic Coordinates¶

O   0.0000000000    0.0000000000    0.1068300000
H   0.0000000000    0.7851780000   -0.4273190000
H   0.0000000000   -0.7851780000   -0.4273190000

Each line contains one atom.

The format is

Element   X-coordinate   Y-coordinate   Z-coordinate

where the coordinates are given in Ångström by default.

The first line defines the oxygen atom, followed by the coordinates of the two hydrogen atoms.

These Cartesian coordinates describe the equilibrium geometry of the water molecule used throughout this tutorial.

Geometry Terminator¶

A single asterisk marks the end of the molecular geometry.

ORCA reads all atomic coordinates until it encounters this symbol. Every input file containing Cartesian coordinates must end with this terminating asterisk.

Complete Input Summary¶

The complete RHF input file performs the following tasks:

Uses the Restricted Hartree–Fock method.
Employs the def2-SVP basis set.
Runs on a single processor core.
Reads the Cartesian geometry of a neutral water molecule.
Starts an SCF procedure to obtain the converged Hartree–Fock wavefunction.

Expected Output Files¶

After a successful calculation, ORCA generates several files. The most important ones are:

File	Description
`job.out`	Main output containing SCF iterations, energies, molecular orbitals, and convergence information
`job.gbw`	Binary wavefunction file used for restart calculations and visualization
`job.ges`	Geometry information used internally by ORCA
`job.property.txt` (optional)	Molecular properties generated by certain calculations

The output file (job.out) is the primary source of information and will be examined in the next section of this tutorial.