# PSIMRCC

Performs multireference coupled cluster computations. This theory should be used only by advanced users with a good working knowledge of multireference techniques.

## General

### CC_NUM_THREADS

> Number of threads

> - Type: integer
> - Default: 1

### CORR_ANSATZ

> The ansatz to use for MRCC computations

> - Type: string
> - Possible Values: SR, MK, BW, APBW
> - Default: MK

### CORR_CCSD_T

> The type of CCSD(T) computation to perform

> - Type: string
> - Possible Values: STANDARD, PITTNER
> - Default: STANDARD

### CORR_CHARGE

> The molecular charge of the target state

> - Type: integer
> - Default: 0

### CORR_MULTP

> The multiplicity, \(M_S(M_S+1)\), of the target state. Must be specified if different from the reference \(M_s\).

> - Type: integer
> - Default: 1

### CORR_WFN

> The type of correlated wavefunction

> - Type: string
> - Possible Values: PT2, CCSD, MP2-CCSD, CCSD_T
> - Default: CCSD

### COUPLING

> The order of coupling terms to include in MRCCSDT computations

> - Type: string
> - Possible Values: NONE, LINEAR, QUADRATIC, CUBIC
> - Default: CUBIC

### COUPLING_TERMS

> Do include the terms that couple the reference determinants?

> - Type: boolean
> - Default: true

### DAMPING_PERCENTAGE

> The amount (percentage) of damping to apply to the amplitude updates. 0 will result in a full update, 100 will completely stall the update. A value around 20 (which corresponds to 20% of the amplitudes from the previous iteration being mixed into the current iteration) can help in cases where oscillatory convergence is observed.

> - Type: double
> - Default: 0.0

### DIAGONALIZE_HEFF

> Do diagonalize the effective Hamiltonian?

> - Type: boolean
> - Default: false

### DIAGONAL_CCSD_T

> Do include the diagonal corrections in (T) computations?

> - Type: boolean
> - Default: true

### DIIS_MAX_VECS

> Maximum number of error vectors stored for DIIS extrapolation

> - Type: integer
> - Default: 7

### DIIS_START

> The number of DIIS vectors needed before extrapolation is performed

> - Type: integer
> - Default: 2

### E_CONVERGENCE

> Convergence criterion for energy. See Table Post-SCF Convergence for default convergence criteria for different calculation types.

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

### FAVG_CCSD_T

> Do use the averaged Fock matrix over all references in (T) computations?

> - Type: boolean
> - Default: false

### FOLLOW_ROOT

> Which root of the effective hamiltonian is the target state?

> - Type: integer
> - Default: 1

### HEFF4

> Do include the fourth-order contributions to the effective Hamiltonian?

> - Type: boolean
> - Default: true

### HEFF_PRINT

> Do print the effective Hamiltonian?

> - Type: boolean
> - Default: false

### LOCK_SINGLET

> Do lock onto a singlet root?

> - Type: boolean
> - Default: false

### MAXITER

> Maximum number of iterations to determine the amplitudes

> - Type: integer
> - Default: 100

### MP2_CCSD_METHOD

> How to perform MP2_CCSD computations

> - Type: string
> - Possible Values: I, IA, II
> - Default: II

### MP2_GUESS

> Do start from a MP2 guess?

> - Type: boolean
> - Default: true

### NO_SINGLES

> Do disregard updating single excitation amplitudes?

> - Type: boolean
> - Default: false

### OFFDIAGONAL_CCSD_T

> Do include the off-diagonal corrections in (T) computations?

> - Type: boolean
> - Default: true

### PT_ENERGY

> The type of perturbation theory computation to perform

> - Type: string
> - Default: SECOND_ORDER

### R_CONVERGENCE

> Convergence criterion for amplitudes (residuals).

> - Type: conv double
> - Default: 1e-9

### SMALL_CUTOFF

> - Type: integer
> - Default: 0

### TIKHONOW_MAX

> The cycle after which Tikhonow regularization is stopped. Set to zero to allow regularization in all iterations

> - Type: integer
> - Default: 5

### TIKHONOW_OMEGA

> The shift to apply to the denominators, {it c.f.} Taube and Bartlett, JCP, 130, 144112 (2009)

> - Type: double
> - Default: 0.0

### TRIPLES_ALGORITHM

> The type of algorithm to use for (T) computations

> - Type: string
> - Possible Values: SPIN_ADAPTED, RESTRICTED, UNRESTRICTED
> - Default: RESTRICTED

### TRIPLES_DIIS

> Do use DIIS extrapolation to accelerate convergence for iterative triples excitations?

> - Type: boolean
> - Default: false

### USE_SPIN_SYM

> Do use symmetry to map equivalent determinants onto each other, for efficiency?

> - Type: boolean
> - Default: true

### WFN_SYM

> The symmetry of the target wavefunction, specified either by Schönflies symbol, or irrep number (in Cotton ordering)

> - Type: string
> - Default: 1

### ZERO_INTERNAL_AMPS

> Do zero the internal amplitudes, i.e., those that map reference determinants onto each other?

> - Type: boolean
> - Default: true

## Expert

### PERTURB_CBS

> Do compute the perturbative corrections for basis set incompleteness?

> - Type: boolean
> - Default: false

### PERTURB_CBS_COUPLING

> Do include the terms that couple different reference determinants in perturbative CBS correction computations?

> - Type: boolean
> - Default: true

### TIKHONOW_TRIPLES

> Do use Tikhonow regularization in (T) computations?

> - Type: boolean
> - Default: false

### USE_SPIN_SYMMETRY

> Whether to use spin symmetry to map equivalent configurations onto each other, for efficiency

> - Type: boolean
> - Default: true