# MRCC¶

Interface to MRCC program written by Mihály Kállay.

## General¶

### E_CONVERGENCE¶

Convergence criterion for energy. See Table Post-SCF Convergence for default convergence criteria for different calculation types. This becomes tol (option #16) in fort.56.

### INTS_TOLERANCE¶

Schwarz screening threshold. Mininum absolute value below which TEI are neglected.

### MRCC_LEVEL¶

Maximum excitation level. This is used ONLY if it is explicitly set by the user. Single-reference case: all excitations up to this level are included, e.g., 2 for CCSD, 3 for CCSDT, 4 for CCSDTQ, etc. This becomes ex.lev (option #1) in fort.56.

• Type: integer

• Default: 2

### MRCC_NUM_DOUBLET_ROOTS¶

Number of root in case of open shell system. This becomes ndoub (option #13) int fort.56.

• Type: integer

• Default: 0

### MRCC_NUM_SINGLET_ROOTS¶

Number of singlet roots. (Strictly speaking number of of roots with M_s=0 and S is even.) Use this option only with closed shell reference determinant, it must be zero otherwise. This becomes nsing (option #2) in fort.56.

• Type: integer

• Default: 1

### MRCC_NUM_TRIPLET_ROOTS¶

Number of triplet roots. (Strictly speaking number of of roots with $$M_s=0$$ and S is odd.) See notes at option MRCC_NUM_SINGLET_ROOTS This becomes ntrip (option #3) in fort.56.

• Type: integer

• Default: 0

## Expert¶

### MRCC_METHOD¶

If more than one root is requested and calc=1, LR-CC (EOM-CC) calculation is performed automatically for the excited states. This overrides all automatic determination of method and will only work with energy(). This becomes CC/CI (option #5) in fort.56. See Table MRCC_METHOD for details.

• Type: integer

• Default: 1

Sets the OMP_NUM_THREADS environment variable before calling MRCC. If the environment variable OMP_NUM_THREADS is set prior to calling Psi4 then that value is used. When set, this option overrides everything. Be aware the -n command-line option described in section Threading does not affect MRCC.

• Type: integer

• Default: 1

### MRCC_RESTART¶

The program restarts from the previously calculated parameters if it is 1. In case it is 2, the program executes automatically the lower-level calculations of the same type consecutively (e.g., CCSD, CCSDT, and CCSDTQ if CCSDTQ is requested) and restarts each calculation from the previous one (rest=2 is available only for energy calculations). Currently, only a value of 0 and 2 are supported. This becomes rest (option #4) in fort.56.

• Type: integer

• Default: 0