Performs configuration interaction (CI) computations of various types, including restricted-active-space (RAS) CI, full CI, the CI component of multi-configuration self-consistent-field (MCSCF) and complete-active-space self-consistent-field (CASSCF) computations, and arbitrary-order perturbation theory and arbitrary-order coupled-cluster computations for small molecules.

Array giving the root numbers of the states to average in a state-averaged procedure such as SA-CASSCF. Root numbering starts from 1.

Type: arrayDefault: No Default

Array giving the weights for each state in a state-averaged procedure

Type: arrayDefault: No Default

Do print a summary of the CI blocks?

Type:booleanDefault: false

Number of threads for DETCI.

Type: integerDefault: 1

Do freeze core orbitals?

Type:booleanDefault: true

Convergence criterion for energy.

Type:conv doubleDefault: 1e-6

Specifies how to handle buffering of CI vectors. A value of 0 makes the program perform I/O one RAS subblock at a time; 1 uses entire CI vectors at a time; and 2 uses one irrep block at a time. Values of 0 or 2 cause some inefficiency in the I/O (requiring multiple reads of the C vector when constructing H in the iterative subspace if

DIAG_METHOD= SEM), but require less core memory.

Type: integerDefault: 1

Do stop DETCI after string information is formed and before integrals are read?

Type:booleanDefault: false

Number of important determinants to print

Type: integerDefault: 20

Convergence criterion for CI residual vector in the Davidson algorithm (RMS error). The default is 1e-4 for energies and 1e-7 for gradients.

Type:conv doubleDefault: 1e-4

In a RAS CI, this is the additional excitation level for allowing electrons out of RAS I into RAS II. The maximum number of holes in RAS I is therefore

EX_LEVEL+ VAL_EX_LEVEL.

Type: integerDefault: 0

An array giving the number of active orbitals (occupied plus unoccupied) per irrep (shorthand to make MCSCF easier to specify than using RAS keywords)

Type: arrayDefault: No Default

maximum number of alpha electrons in RAS III

Type: integerDefault: -1

maximum number of beta electrons in RAS III

Type: integerDefault: -1

An array giving the number of restricted doubly-occupied orbitals per irrep (not excited in CI wavefunctions, but orbitals can be optimized in MCSCF)

Type: arrayDefault: No Default

An array giving the number of restricted unoccupied orbitals per irrep (not occupied in CI wavefunctions, but orbitals can be optimized in MCSCF)

Type: arrayDefault: No Default

The value of the spin quantum number is given by this option. The default is determined by the value of the multiplicity. This is used for two things: (1) determining the phase of the redundant half of the CI vector when the component is used (i.e.,

MS0=TRUE), and (2) making sure the guess vector has the desired value of (ifS_SQUAREDisTRUEandICORE=1).

Type: doubleDefault: 0.0

This specifies which method is to be used in diagonalizing the Hamiltonian. The valid options are:

RSP, to form the entire H matrix and diagonalize using libciomr to obtain all eigenvalues (n.b. requires HUGE memory);OLSEN, to use Olsen’s preconditioned inverse subspace method (1990);MITRUSHENKOV, to use a 2x2 Olsen/Davidson method; andDAVIDSON(orSEM) to use Liu’s Simultaneous Expansion Method, which is identical to the Davidson method if only one root is to be found. There also exists a SEM debugging mode,SEMTEST. TheSEMmethod is the most robust, but it also requires CI vectors on disk, where is the maximum number of iterations and is the number of roots.

Type: stringPossible Values: RSP, OLSEN, MITRUSHENKOV, DAVIDSON, SEM, SEMTESTDefault: SEM

Do use least-squares extrapolation in iterative solution of CI vector?

Type:booleanDefault: false

Number of iterations between least-squares extrapolations

Type: integerDefault: 3

Minimum converged energy for least-squares extrapolation to be performed

Type:conv doubleDefault: 3

This specifies the type of preconditioner to use in the selected diagonalization method. The valid options are:

DAVIDSONwhich approximates the Hamiltonian matrix by the diagonal elements;H0BLOCK_INVwhich uses an exact Hamiltonian ofH0_BLOCKSIZEand explicitly inverts it;GEN_DAVIDSONwhich does a spectral decomposition of H0BLOCK;ITER_INVusing an iterative approach to obtain the correction vector of H0BLOCK. TheH0BLOCK_INV,GEN_DAVIDSON, andITER_INVapproaches are all formally equivalent but theITER_INVis less computationally expensive. Default isDAVIDSON.

Type: stringPossible Values: LANCZOS, DAVIDSON, GEN_DAVIDSON, H0BLOCK, H0BLOCK_INV, ITER_INV, H0BLOCK_COUPLING, EVANGELISTIDefault: DAVIDSON

Do write the natural orbitals?

Type:booleanDefault: false

Sets the root number for which CI natural orbitals are written to PSIF_CHKPT. The default value is 1 (lowest root).

Type: integerDefault: 1

Do compute one-particle density matrix if not otherwise required?

Type:booleanDefault: false

Do average the OPDM over several roots in order to obtain a state-average one-particle density matrix? This density matrix can be diagonalized to obtain the CI natural orbitals.

Type:booleanDefault: false

Do print the one-particle density matrix for each root?

Type:booleanDefault: false

Do compute the transition density? Note: only transition densities between roots of the same symmetry will be evaluated. DETCI does not compute states of different irreps within the same computation; to do this, lower the symmetry of the computation.

Type:booleanDefault: false

Do compute two-particle density matrix if not otherwise required?

Type:booleanDefault: false

Do print the two-particle density matrix? (Warning: large tensor)

Type:booleanDefault: false

The root to write out the two-particle density matrix for (the one-particle density matrices are written for all roots). Useful for a state-specific CASSCF or CI optimization on an excited state.

Type: integerDefault: 1

Gives the number of vectors to retain when the Davidson subspace is collapsed (see

MAX_NUM_VECS. If greater than one, the collapsed subspace retains the best estimate of the CI vector for the previous n iterations. Defaults to 1.

Type: integerDefault: 1

Maximum number of Davidson subspace vectors which can be held on disk for the CI coefficient and sigma vectors. (There is one H(diag) vector and the number of D vectors is equal to the number of roots). When the number of vectors on disk reaches the value of MAX_NUM_VECS, the Davidson subspace will be collapsed to

COLLAPSE_SIZEvectors for each root. This is very helpful for saving disk space. Defaults toMAXITER*NUM_ROOTS+NUM_INIT_VECS

Type: integerDefault: 0

Number of vectors to export

Type: integerDefault: 1

Do store converged vector(s) at the end of the run? The vector(s) is(are) stored in a transparent format such that other programs can use it easily. The format is specified in psi4/src/lib/libqt/slaterdset.h .

Type:booleanDefault: false

Do compute the MPn series out to kth order where k is determined by

MAX_NUM_VECS? For open-shell systemsREFERENCEis ROHF,WFNis ZAPTN), DETCI will compute the ZAPTn series.GUESS_VECTORmust be set to UNIT,HD_OTFmust be set to TRUE, andHD_AVGmust be set to orb_ener; these should happen by default for MPN = TRUE.

Type:booleanDefault: false

maximum number of alpha electrons in RAS III, for CC

Type: integerDefault: -1

maximum number of beta electrons in RAS III, for CC

Type: integerDefault: -1

The CC excitation level

Type: integerDefault: 2

maximum number of electrons in RAS III + IV, for CC

Type: integerDefault: -1

maximum number of electrons in RAS III, for CC

Type: integerDefault: -1

maximum number of electrons in RAS IV, for CC

Type: integerDefault: -1

The CC valence excitation level

Type: integerDefault: 0

Do import a CC vector from disk?

Type:booleanDefault: false

Do export a CC vector to disk?

Type:booleanDefault: false

How often to do a DIIS extrapolation. 1 means do DIIS every iteration, 2 is every other iteration, etc.

Type: integerDefault: 1

Maximum number of error vectors stored for DIIS extrapolation

Type: integerDefault: 5

Minimum number of error vectors stored for DIIS extrapolation

Type: integerDefault: 2

Iteration at which to start using DIIS

Type: integerDefault: 1

Number of important CC amplitudes per excitation level to print. CC analog to

NUM_DETS_PRINT

Type: integerDefault: 10

Do print the sigma overlap matrix? Not generally useful.

Type:booleanDefault: false

An array of length

EX_LEVELspecifying whether each excitation type (S,D,T, etc.) is allowed (1 is allowed, 0 is disallowed). Used to specify non-standard CI spaces such as CIST.

Type: arrayDefault: No Default

Do restrict strings with in RAS IV? Useful to reduce the number of strings required if MIXED4=true, as in a split-virutal CISD[TQ] computation. If more than one electron is in RAS IV, then the holes in RAS I cannot exceed the number of particles in RAS III + RAS IV (i.e.,

EX_LEVEL, or else the string is discarded.

Type:booleanDefault: false

Do eliminate determinants not valid for spin-complete spin-flip CI’s? [see J. S. Sears et al, J. Chem. Phys. 118, 9084-9094 (2003)]

Type:booleanDefault: false

This parameter specifies the size of the H0 block of the Hamiltonian which is solved exactly. The n determinants with the lowest SCF energy are selected, and a submatrix of the Hamiltonian is formed using these determinants. This submatrix is used to accelerate convergence of the CI iterations in the OLSEN and MITRUSHENKOV iteration schemes, and also to find a good starting guess for the SEM method if

GUESS_VECTORisH0_BLOCK. Defaults to 400. Note that the program may change the given size for Ms=0 casesMS0is TRUE) if it determines that the H0 block includes only one member of a pair of determinants related by time reversal symmetry. For very small block sizes, this could conceivably eliminate the entire H0 block; the program should print warnings if this occurs.

Type: integerDefault: 400

Do use coupling block in preconditioner?

Type:booleanDefault: false

Parameters which specifies the size of the coupling block within the generalized davidson preconditioner.

Type: integerDefault: 0

size of H0 block for initial guess

Type: integerDefault: 400

How to average H diag energies over spin coupling sets.

HD_EXACTuses the exact diagonal energies which results in expansion vectors which break spin symmetry.HD_KAVEaverages the diagonal energies over a spin-coupling set yielding spin pure expansion vectors.ORB_ENERemploys the sum of orbital energy approximation giving spin pure expansion vectors but usually doubles the number of Davidson iterations.EVANGELISTIuses the sums and differences of orbital energies with the SCF reference energy to produce spin pure expansion vectors.LEININGERapproximation which subtracts the one-electron contribution from the orbital energies, multiplies by 0.5, and adds the one-electron contribution back in, producing spin pure expansion vectors and developed by Matt Leininger and works as well asEVANGELISTI.

Type: stringPossible Values: EVANGELISTI, HD_EXACT, HD_KAVE, ORB_ENER, LEININGER, Z_KAVEDefault: EVANGELISTI

In following a particular root (see

FOLLOW_ROOT, sometimes the root number changes. To follow a root of a particular character, one can specify a list of determinants and their coefficients, and the code will follow the root with the closest overlap. The user specifies arrays containing the absolute alpha string indices (A_i below), absolute beta indices (B_i below), and CI coefficients (C_i below) to form the desired vector. The format is FOLLOW_VECTOR = [ [[A_1, B_1], C_1], [[A_2, B_2], C_2], ...].

Type: arrayDefault: No Default

Do invoke the FILTER_GUESS options that are used to filter out some trial vectors which may not have the appropriate phase convention between two determinants? This is useful to remove, e.g., delta states when a sigma state is desired. The user inputs two determinants (by giving the absolute alpha string number and beta string number for each), and also the desired phase between these two determinants for guesses which are to be kept. FILTER_GUESS = TRUE turns on the filtering routine. Requires additional keywords

FILTER_GUESS_DET1FILTER_GUESS_DET2andFILTER_GUESS_SIGN

Type:booleanDefault: false

Array specifying the absolute alpha string number and beta string number for the first determinant in the filter procedure. (See

FILTER_GUESS.

Type: arrayDefault: No Default

Array specifying the absolute alpha string number and beta string number for the second determinant in the filter procedure. (See

FILTER_GUESS.

Type: arrayDefault: No Default

The required phase (1 or -1) between the two determinants specified by

FILTER_GUESS_DET1andFILTER_GUESS_DET2

Type: integerDefault: 1

If present, the code will try to filter out a particular determinant by setting its CI coefficient to zero. FILTER_ZERO_DET = [alphastr, betastr] specifies the absolute alpha and beta string numbers of the target determinant. This could be useful for trying to exclude states that have a nonzero CI coefficient for the given determinant. However, this option was experimental and may not be effective.

Type: arrayDefault: No Default

Guess vector type. Accepted values are

UNITfor a unit vector guessNUM_ROOTSandNUM_INIT_VECSmust both be 1);H0_BLOCKto use eigenvectors from the H0 BLOCK submatrix (default);DFILEto use NUM_ROOTS previously converged vectors in the D file;IMPORTto import a guess previously exported from a CI computation (possibly using a different CI space)

Type: stringPossible Values: UNIT, H0_BLOCK, DFILE, IMPORTDefault: H0_BLOCK

The number of initial vectors to use in the CI iterative procedure. Defaults to the number of roots.

Type: integerDefault: 0

Irrep for CI vectors; -1 = find automatically. This option allows the user to look for CI vectors of a different irrep than the reference. This probably only makes sense for Full CI, and it would probably not work with unit vector guesses. Numbering starts from zero for the totally-symmetric irrep.

Type: integerDefault: -1

If 0, save the MPn energy; if 1, save the MP(2n-1) energy (if available from

MPN_WIGNER= true); if 2, save the MP(2n-2) energy (if available fromMPN_WIGNER= true).

Type: integerDefault: 0

Do employ an orthonormal vector space rather than storing the kth order wavefunction?

Type:booleanDefault: false

Do use Wigner formulas in the series?

Type:booleanDefault: true

The magnitude of perturbation in

Type: doubleDefault: 1.0

Do fix amplitudes involving RAS I or RAS IV? Useful in mixed MP2-CC methods.

Type:booleanDefault: false

Number of external indices before amplitude gets fixed by

CC_FIX_EXTERNALExperimental.

Type: integerDefault: 1

CC_MACRO = [ [ex_lvl, max_holes_I, max_parts_IV, max_I+IV], [ex_lvl, max_holes_I, max_parts_IV, max_I+IV], ... ] Optional additional restrictions on allowed exictations in coupled-cluster computations, based on macroconfiguration selection. For each sub-array, [ex_lvl, max_holes_I, max_parts_IV, max_I+IV], eliminate cluster amplitudes in which: [the excitation level (holes in I + II) is equal to ex_lvl] AND [there are more than max_holes_I holes in RAS I, there are more than max_parts_IV particles in RAS IV, OR there are more than max_I+IV quasiparticles in RAS I + RAS IV].

Type: arrayDefault: No Default

Do ignore block if num holes in RAS I and II is cc_ex_lvl and if any indices correspond to RAS I or IV (i.e., include only all-active higher excitations)?

Type:booleanDefault: true

Do update T amplitudes with orbital eigenvalues? (Usually would do this). Not doing this is experimental.

Type:booleanDefault: true

Do use variational energy expression in CC computation? Experimental.

Type:booleanDefault: false

Do use some routines based on the papers of Bendazzoli et al. to calculate sigma? Seems to be slower and not worthwhile; may disappear eventually. Works only for full CI and I don’t remember if I could see how their clever scheme might be extended to RAS in general.

Type:booleanDefault: false

Do store strings specifically for FCI? (Defaults to TRUE for FCI.)

Type:booleanDefault: false

Do string replacements on the fly in DETCI? Can save a gigantic amount of memory (especially for truncated CI’s) but is somewhat flaky and hasn’t been tested for a while. It may work only works for certain classes of RAS calculations. The current code is very slow with this option turned on.

Type:booleanDefault: false