# CI calculation¶

If you have not already done so, you should take a look at the eT_launch page to get an understanding of how the launch script works.

## Setting up a FCI calculation¶

A Full Configuration Interaction (FCI) calculation is performed by specifying a HF (`hf`) or ROHF (`rohf`) wave function and the FCI wave function (`fci`) in the method section of the input file. E.g., for a RHF calculation we write

```- method
hf
fci
```

To perform a single-point FCI calculation it is necessary to specify `ground state` do section:

```- do
ground state
```

The FCI calculations require the solver ci section in which it is necessary to specify the number of states you want to compute in your calculation, for instance:

```- solver ci
states: 4
```

It is also possible to compute the FCI dipole and/or quadrupole moments for the ground state by including the ci mean value section the `dipole` and/or `quadrupole` keyword

```- ci mean value
dipole
```

It is also possible to compute FCI transition dipole and/or quadrupole moments by including the ci transition property the `dipole` and/or `quadrupole` keyword together with the `initial states` and `final states` keywords to specify the states involved in the transition.

```- fci transition property
dipole
initial states: {1}
final states: {2}
```

A minimal working example for an FCI calculation is:

```- do
ground state

- memory
available: 16

- method
hf
fci

- solver ci
states: 4
```

Note

For open-shell calculations you might want to specify the multiplicity in the system section. The default multiplicity is `1`.

In the output file, after the iterations of the FCI solver

```Iteration:                 20
Reduced space dimension:   39

Root  Eigenvalue (Re)   Eigenvalue (Im)    |residual|   Delta E (Re)
--------------------------------------------------------------------
1 -76.621857997622    0.000000000000    0.4449E-10   0.1137E-12
2 -76.590840482570    0.000000000000    0.2925E-10   0.2842E-12
3 -76.504315134867    0.000000000000    0.7237E-09   0.7105E-13
4 -76.464509592995    0.000000000000    0.2011E-07   0.0000E+00
--------------------------------------------------------------------
```

you will find the energies, the spin multiplicity and the largest CI amplitudes for the desired states:

```Electronic state nr. 1

Energy (Hartree):     -76.621857997622
Spin Multiplicity:      3.000000000000

Largest CI amplitudes:
----------------------------
0ab22222     -0.694844436546
0ba22222      0.694844436546
2ab20222      0.052140010314
2ba20222     -0.052140010313
abb2a222      0.048584038655
baa2b222     -0.048584038652
b022a222      0.048242124437
a022b222     -0.048242124435
b202a222     -0.034631052738
a202b222      0.034631052736
----------------------------
```

The amplitudes are labeled by the occupation of the molecular orbitals where 0 means unoccupied, a occupied by alpha electron, b occupied by beta electron and 2 means doubly occupied. The occupation of the highest energy MO is defined by the first number and the occupation of the lowest energy MO by the right most “digit”. After the amplitudes summary, you will find a summary of the energies of the computed states

```- Electronic energies:

Energy
------------------------------------------
State                (Hartree)             (eV)
---------------------------------------------------------------
1                -76.621857997622    -2084.986955542918
2                -76.590840482570    -2084.142925967466
3                -76.504315134866    -2081.788451330578
4                -76.464509592995    -2080.705287364980
---------------------------------------------------------------
eV/Hartree (CODATA 2014): 27.21138602
```

If you requested a mean value calculation for the ground state, after the energy summary you will find the information about the ground state dipole and/or quadrupole moment

## Setting up a CASCI calculation¶

It is also possible to perform FCI calculations only in an active space of canonical molecular orbitals, leading to a Complete Active Space Configuration Interaction (CASCI) calculation.

To perform a CASCI calculation the keyword fci needs to be exchanged with casci in the method section.

```- method
hf
casci
```

Additionally, it is necessary to specify the number of active electrons and active canonical orbitals included in the active space. This is done using the canonical keyword in the active space section. For a CASCI calculation with 2 active electrons in 4 active canonical orbitals we write

```- active space
canonical: {2, 4}
```

## Relevant input sections¶

System section

CI solver section

CI mean value section

CI transition property section

Active space section