# Restarting X2C calculations¶

Here we will show how one can run a Hartree–Fock (HF) calculation in
2-component mode with the exact two component (X2C) Hamiltonian and restart a
property calculation at the HF level in a second run without the need to re-do
the X2C transformation. For the initial SCF step we use the following input
file (copy the content below into the `scf_x2c.inp`

file):

```
**DIRAC
.WAVE FUNCTION
**MOLECULE
*BASIS
! Use the same basis for all atoms
.DEFAULT
dyall.av3z
**HAMILTONIAN
! Use the exact two-component Hamiltonian
! with atomic mean field spin-orbit
.X2C
**INTEGRALS
*READIN
! A limitation in the AMFI code requires an uncontracted basis
.UNCONTRACT
**WAVE FUNCTION
.SCF
*END OF
```

together with the following molecule file (copy it into the `br2.xyz`

file):

```
2
Br 0.0 0.0 1.244
Br 0.0 0.0 -1.244
```

This molecule has linear symmetry (Dinf,h), and the program will detect this automatically. In most parts of the program the highest abelian subgroup (D2h) will be used instead of the full linear symmetry. In this example we use Dyall’s augmented TZ basis dyall.av3z in uncontracted form.

Prior to the HF calculation, the Hamiltonian transformation from 4- to
2-components will be performed and for restart purposes one should therefore
save the files `X2CMAT`

and `AOMOMAT`

containing the Hamiltonian
information and the file `DFCOEF`

containing the (converged) MO-coefficients.

We run the initial SCF step using:

```
pam --inp=scf_x2c.inp --mol=br2.xyz --get="X2CMAT AOMOMAT DFCOEF"
```

The output will be written to the file `scf_x2c_br2.out`

, which can be
monitored during the calculation.

In order to proceed in a second step with a property calculation of,
e.g. the contact density at the Br nucleus at the HF level, we use the following
input file (copy and paste it into the `property_x2c.inp`

input file):

```
**DIRAC
!.WAVE FUNCTION commented out to skip SCF step
.PROPERTIES
**MOLECULE
*BASIS
! Use the same basis for all atoms
.DEFAULT
dyall.av3z
**HAMILTONIAN
! Use the exact two-component Hamiltonian
! with atomic mean field spin-orbit
.X2C
**INTEGRALS
*READIN
! A limitation in the AMFI code requires an uncontracted basis
.UNCONTRACT
**WAVE FUNCTION
.SCF
**PROPERTIES
.RHONUC
*END OF
```

Restarting from the (converged) MO-coefficients and reading the transformed X2C Hamiltonian from file is done automatically once the files are present. We therefore restart the calculation using:

```
pam --inp=property_x2c.inp --mol=br2.xyz --put="X2CMAT AOMOMAT DFCOEF"
```

## Restart files¶

AOMOMAT and X2CMAT never change during the SCF iterations, hence you can use them for an SCF restart at the same geometry. If you however change the geometry and restart from those files, all results will be wrong.

AOMOMAT contains amongst other things the S^{+/-1/2} overlap matrix which is needed to transform the Fock operator from AO to orthonormal MO basis before diagonalization. X2CMAT contains the transformed two-component one-electron operator with possible two-electron spin-orbit corrects added as well as all property operators (in AO basis) in proper two-component form. All these quantities are independent of the current solution in an SCF iteration.

Shen X2C is restarted with just:

```
pam --put="DFCOEF"
```

(i.e. not putting AOMOMAT and X2CMAT), the X2C code will recompute AOMOMAT and X2CMAT automatically, which takes just a few extra seconds. The subsequent SCF iterations or property evaluations deliver the same results as if AOMOMAT and X2CMAT had been put to the scratch directory.

An X2C calculation currently cannot be restarted by:

```
pam --fullrestart="ARCHIVE.tgz"
```