# Getting started with DIRAC¶

You have installed and tested DIRAC and now it’s time to run your first DIRAC calculation!

You have two possibilities to run DIRAC. The traditional uses two input files: the *.inp file, which determines what should be calculated, together with the *.mol file, which defines the molecular geometry and the basis set. Alternatively you can use the *.inp file, file together with a geometry file (*.xyz file,). In this case the *.xyz file provides the nuclear coordinates and the *.inp file contains in addition to the job description also the basis set information (see **MOLECULE).

DIRAC writes its output to a text file, named after the molecule and input file names. If this file already exists, for example from a previous calculation, the old file is renamed to make place for the new output. The python script pam that launches the main DIRAC executable also writes some general information to standard output.

## Dirac-Coulomb SCF¶

Let’s try it out and run our first DIRAC calculation with the following minimal input (hf.inp) to calculate the Dirac-Coulomb Hartree-Fock ground state (with the default simple Coulombic correction to approximate the contribution from (SS|SS) integrals:

**DIRAC
.WAVE FUNCTION
**WAVE FUNCTION
.SCF
**MOLECULE
*BASIS
.DEFAULT
cc-pVDZ
*END OF INPUT

together with the following example geometry file (methanol.xyz):

6
my first DIRAC calculation # anything can be in this line
C       0.000000000000       0.138569980000       0.355570700000
O       0.000000000000       0.187935770000      -1.074466460000
H       0.882876920000      -0.383123830000       0.697839450000
H      -0.882876940000      -0.383123830000       0.697839450000
H       0.000000000000       1.145042790000       0.750208830000
H       0.000000000000      -0.705300580000      -1.426986340000

Now start the calculation by executing:

pam --mol=methanol.xyz --inp=hf.inp


If everything works fine the results of the calculation will be written to hf_methanol.out. In addition an archive file hf_methanol.tgz will be created. Depending on the type of calculation the archive file may include:

• DFCOEF containing molecular orbital coefficients and possibly energies.
• cube files containing plots of molecular properties, typically electron densities, in the gridded Gaussian Cube format.
• Additional files for restarting particular types of calculations.

## Closed shell CCSD¶

Running CCSD(T) calculations is straightforward if the ground state of the molecule can be well-described by a single closed shell determinant. This is the case for many molecules.

The input requires the specification of a basis set (TZ or better is recommended, but for this example we will take DZ to reduce the run time). We take the inter halogen molecule ClF as an example and use the default cut-offs for correlating electrons (include all valence electrons with energy above -10 hartree) and truncation of virtual space (delete virtuals above 20 hartree). The geometry file (clf.xyz) is very simple and only requires to specify the coordinates. The program will then identify the symmetry as $$C_{\infty v}$$:

2
ClF molecule at equilibrium distance taken from NIST
Cl   0.0  0.0  0.0
F    0.0  0.0  1.628

We specify the wave function type (SCF, followed by RELCCSD) and basis set in the input file (cc.inp) to calculate the CCSD(T) energy with the Dirac-Coulomb Hamiltonian again with the contribution from (SS|SS) integrals approximated by a simple Coulombic correction:

**DIRAC
.WAVE FUNCTION
**WAVE FUNCTION
.SCF
.RELCCSD
**MOLECULE
*BASIS
.DEFAULT
cc-pVDZ
*END OF INPUT

Now start the calculation by executing:

pam --mol=clf.xyz --inp=cc.inp


If everything works fine the results of the calculation will be written to cc_clf.out. In addition an archive file cc_clf.tgz will be created, as mentioned above.

You can suppress creation of the archive file by:

pam --noarch