**CAS_INPUT**: specifies how the input for a CASSCF calculation is provided. Options:

a) **ORBITALS (=0, default)**: a list of orbital numbers, as printed after the SCF solution, is provided as a list of integers separated by spaces.

b) **OCCUPATION (=1)**: the number of internal (i.e., doubly occupied) orbitals is provided for each irrep as a list of integers separated by one space and, *on a new line*, the list of active orbitals is provided for each irrep, as a list of integers separated by one space

**CAS_GUESS**: specifies what kind of guess to use for the CASSCF wavefunction.

a) **CANONICAL (=0, default)**: use the Hartree-Fock wavefunction as a guess. For UHF orbitals, this corresponds to using alpha orbitals only.

b) **CASCI (=1)**: starting from the Hartree-Fock orbitals, compute a CAS-CI wavefunction and use it as a guess. This might help convergence.

c) **MODCASCI (=2)**: experimental use.

d) **CORECASCI (=3)**: computes a CASCI guess using the core orbitals.

**CAS_PRINT=XY**: printing flag. Y=1 prints information on the microiterations convergence, X=1,2,3 prints (more and more detailed) timing informations at each iteration.**CAS_ITMAX=N**: Max number of macroiterations. Default is 50, which should be plenty.**CAS_MITMAX=N**: Max number of microiterations. Default is 100**CAS_MMAX=M**: largest allowed subspace size for the microiterations, which require a Davidson-like diagonalization**CAS_GECCO**: (ON/OFF) specifies whether to set up the files for a subsequent internally contracted MRCC calculation using the GeCCo code by A. Koehn and M. Hanhauer. Default: OFF (=0)

**Advanced options**

The following keywords control the algorithm used to solve the microiterations and some internal parameters and should be used with caution

**QC_RTRUST=X**: X/1000 specifies the starting trust radius for the adaptive restricted-step second order optimization procedure. Default: 700**CAS_MICRO**: specifies what preconditioner to use for the microiterations. The options are

a) **ILU/JACOBI (=0, default)**: use the incomplete LU decomposition (ILU) to precondition the Davidson iterations for the MO part. If the Jacobi-Davidson algorithm has been chosen, use the standard diagonal preconditioner for the first iterations and the ILU one to precondition the GMRES solution to the Jacobi-Davidson linear equations

b) **JACOBI (=1)**: use the standard Davidson diagonalization, using a diagonal (i.e., Jacobi) preconditioner

c) **ILU (=2)**: use the incomplete LU decomposition of the MO rotation Hessian to precondition the orbital updates in the microiterations. This is cheap and effective and is used as a default

d) **EIG(=3)**: Explicitly diagonalize the MO rotation Hessian. This option is mainly for debugging purposes, but might be used **with caution** for tough cases.

**CAS_JACDAV**: (ON/OFF) specifies whether the Jacobi-Davidson (JD) scheme should be used instead than Davidson for the microiterations. Either the ILU (default) or EIG (with CAS_MICRO=EIG) preconditioner is used to solve the linear equations. JD is an expensive, yet very powerful method to find one or few eigenvectors of a matrix. It can be used for very difficult cases.**CAS_MICMIN=N**: the minimum (loosest) convergence criterion to be used for the microiterations is set to 10^{-N}. Default: 2**CAS_MICMAX=N**: the maximum (tightest) convergence criterion to be used for the microiterations is set to 10^{-N}. Default: 10

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