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# 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

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.

Methods available in automatic interface with MRCC
CCSD | CCSD(T) [1] | CCSD(T)_L [1]
CCSDT | CCSDT(Q) [1] | CCSDT(Q)_L [1]
CCSDTQ | CCSDTQ(P) [1] | CCSDTQ(P)_L [1]
CCSDTQP | CCSDTQP(H) [1] | CCSDTQP(H)_L [1]
CCSDTQPH |  |
CCSDT-1a | CCSDT-1b | CCSDT-3
CCSDTQ-1a | CCSDTQ-1b | CCSDTQ-3
CCSDTQP-1a | CCSDTQP-1b | CCSDTQP-3
CCSDTQPH-1a | CCSDTQPH-1b | CCSDTQPH-3
CC2 |  |
CC3 |  |
CC4 |  |
CC5 |  |
CC6 |  |

[1] | (1, 2, 3, 4, 5, 6, 7, 8) Pertubative methods not available with ROHF reference.

Frozen-core approximation is also supported in the MRCC interface. To optimize CH4 with CCSDT freezing the 1s 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')

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