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# 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.

> - Type: conv double
> - Default: 1e-6

### INTS_TOLERANCE

> Minimum absolute value below which integrals are neglected.

> - Type: conv double
> - Default: 1.0e-12

### MRCC_LEVEL

> Maximum excitation level. This is used ONLY if it is explicity 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 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 .. table:: MRCC methods +——-+————–+————————————————————-+ + Value + 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 + + +——-+————–+————————————————————-+

> - Type: integer
> - Default: 1

### MRCC_OMP_NUM_THREADS

> 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

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