# Interface to MRCC by M. Kállay¶

*Code author: Justin M. Turney and Andrew C. Simmonett*

*Section author: Justin M. Turney*

*Module:* Keywords, PSI Variables, MRCC, Samples

Psi4 contains code to interface to the MRCC program of M. Kállay and J. Gauss. The license and source code of the MRCC program must be obtained from Mihály Kállay (http://www.mrcc.hu/).

## Installation¶

Follow the instructions provided with the source to build the MRCC programs.
To be used by Psi4, ensure that the program binary (`dmrcc`

) can be
found in your `PATH`

. If Psi4 is unable to execute the binary, an
error will be reported.

## Running MRCC¶

MRCC can be invoked in similar fashion as other theories provided in Psi4. For example, if you want to obtain the CCSDT energy for water with cc-pVDZ using MRCC simply provide the following:

```
molecule h2o {
O
H 1 1.0
H 1 1.0 2 104.5
}
set {
basis cc-pVDZ
}
energy('mrccsdt')
```

`'mrccsdt'`

in the call to `energy()`

instructs Psi4 to first
perform an RHF calculation and then call MRCC to compute the CCSDT energy.
For a CCSDT(Q) energy, simply use `'mrccsdt(q)'`

in the call to
`energy()`

. MRCC can be used to perform geometry optimization and
frequency calculations for electronic ground states only.

At this time, Psi4 is only able to automatically generate the proper
input file for MRCC for the methods listed in table below.
To utilize any method described in the table, you must prefix
the method name with `MR`

. For other methods, you will be required to
use the MRCC keywords described in Appendix MRCC.
Note that perturbative methods (`ccsd(t)`

, `ccsdtqp(h)_l`

, etc.)
are not available with REFERENCE ROHF.

name calls method in Kallay’s MRCC program [manual] mrccsd CC through doubles mrccsdt CC through triples mrccsdtq CC through quadruples mrccsdtqp CC through quintuples mrccsdtqph CC through sextuples mrccsd(t) CC through doubles with perturbative triples mrccsdt(q) CC through triples with perturbative quadruples mrccsdtq(p) CC through quadruples with pertubative quintuples mrccsdtqp(h) CC through quintuples with pertubative sextuples mrccsd(t)_l mrccsdt(q)_l mrccsdtq(p)_l mrccsdtqp(h)_l mrccsdt-1a CC through doubles with iterative triples (cheapest terms) mrccsdtq-1a CC through triples with iterative quadruples (cheapest terms) mrccsdtqp-1a CC through quadruples with iterative quintuples (cheapest terms) mrccsdtqph-1a CC through quintuples with iterative sextuples (cheapest terms) mrccsdt-1b CC through doubles with iterative triples (cheaper terms) mrccsdtq-1b CC through triples with iterative quadruples (cheaper terms) mrccsdtqp-1b CC through quadruples with iterative quintuples (cheaper terms) mrccsdtqph-1b CC through quintuples with iterative sextuples (cheaper terms) mrcc2 approximate CC through doubles mrcc3 approximate CC through triples mrcc4 approximate CC through quadruples mrcc5 approximate CC through quintuples mrcc6 approximate CC through sextuples mrccsdt-3 CC through doubles with iterative triples (all but the most expensive terms) mrccsdtq-3 CC through triples with iterative quadruples (all but the most expensive terms) mrccsdtqp-3 CC through quadruples with iterative quintuples (all but the most expensive terms) mrccsdtqph-3 CC through quintuples with iterative sextuples (all but the most expensive terms)

Frozen-core approximation is also supported in the MRCC interface.
To optimize CH_{4} with CCSDT freezing the 1*s* on carbon, run:

```
molecule H2O {
O
H 1 r
H 1 r 2 104.5
r = 1.0
}
set {
basis cc-pVDZ
freeze_core true
}
optimize('mrccsdt')
```

## Interface Details¶

MRCC_METHOD | Method | Description |
---|---|---|

1 | CC | |

2 | CC(n-1)[n] | |

3 | CC(n-1)(n) | (CC(n-1)[n] energy is also calculated) |

4 | CC(n-1)(n)_L | (CC(n-1)[n] and CC(n-1)(n) energies are also calculated) |

5 | CC(n)-1a | |

6 | CC(n)-1b | |

7 | CCn | |

8 | CC(n)-3 |