# DETCI¶

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.

## General Options¶

### AVG_STATES¶

Array giving the root numbers of the states to average in a state-averaged procedure such as SA-CASSCF. Root numbering starts from 0.

Type: arrayDefault: No Default

### AVG_WEIGHTS¶

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

Type: arrayDefault: No Default

### A_RAS3_MAX¶

maximum number of alpha electrons in RAS III

Type: integerDefault: -1

### B_RAS3_MAX¶

maximum number of beta electrons in RAS III

Type: integerDefault: -1

### CIBLKS_PRINT¶

Do print a summary of the CI blocks?

Type: booleanDefault: false

### CI_MAXITER¶

Maximum number of iterations to diagonalize the Hamiltonian

Type: integerDefault: 24

### DETCI_FREEZE_CORE¶

Do freeze core orbitals?

Type: booleanDefault: true

### E_CONVERGENCE¶

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

Type: conv doubleDefault: 1e-6

### ICORE¶

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

### ISTOP¶

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

Type: booleanDefault: false

### NUM_DETS_PRINT¶

Number of important determinants to print

Type: integerDefault: 20

### R_CONVERGENCE¶

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

### S¶

The value of the spin quantum number \(S\) 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 \(M_s = 0\) component is used (i.e., MS0 =

`TRUE`

), and (2) making sure the guess vector has the desired value of \(\langle S^2\rangle\) (if CALC_S_SQUARED is`TRUE`

and ICORE =`1`

).

Type: doubleDefault: 0.0

### VAL_EX_LEVEL¶

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

## Diagonalization Methods¶

### DIAG_METHOD¶

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; and`DAVIDSON`

(or`SEM`

) 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`

. The`SEM`

method is the most robust, but it also requires \(2NM+1\) CI vectors on disk, where \(N\) is the maximum number of iterations and \(M\) is the number of roots.

Type: stringPossible Values: RSP, DAVIDSON, SEMDefault: SEM

### LSE¶

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

Type: booleanDefault: false

### LSE_COLLAPSE¶

Number of iterations between least-squares extrapolations

Type: integerDefault: 3

### LSE_TOLERANCE¶

Minimum converged energy for least-squares extrapolation to be performed

Type: conv doubleDefault: 3

### PRECONDITIONER¶

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

`DAVIDSON`

which approximates the Hamiltonian matrix by the diagonal elements;`H0BLOCK_INV`

which uses an exact Hamiltonian of H0_BLOCKSIZE and explicitly inverts it;`GEN_DAVIDSON`

which does a spectral decomposition of H0BLOCK;`ITER_INV`

using an iterative approach to obtain the correction vector of H0BLOCK. The`H0BLOCK_INV`

,`GEN_DAVIDSON`

, and`ITER_INV`

approaches are all formally equivalent but the`ITER_INV`

is less computationally expensive. Default is`DAVIDSON`

.

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

## Density Matrices¶

## Root Following¶

### FOLLOW_ROOT¶

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: 0

## Guess Vectors¶

## File Handling¶

### COLLAPSE_SIZE¶

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

### MAX_NUM_VECS¶

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_SIZE vectors for each root. This is very helpful for saving disk space. Defaults to CI_MAXITER * NUM_ROOTS + NUM_INIT_VECS

Type: integerDefault: 0

## General-Order Perturbation Theory¶

### MPN¶

Do compute the MPn series out to kth order where k is determined by MAX_NUM_VECS ? For open-shell systems REFERENCE is ROHF, WFN is ZAPTN), DETCI will compute the ZAPTn series. GUESS_VECTOR must be set to UNIT, HD_OTF must be set to TRUE, and HD_AVG must be set to orb_ener; these should happen by default for MPN = TRUE.

Type: booleanDefault: false

## General-Order Coupled-Cluster¶

### CC_A_RAS3_MAX¶

maximum number of alpha electrons in RAS III, for CC

Type: integerDefault: -1

### CC_B_RAS3_MAX¶

maximum number of beta electrons in RAS III, for CC

Type: integerDefault: -1

### CC_EX_LEVEL¶

The CC excitation level

Type: integerDefault: 2

### CC_RAS34_MAX¶

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

Type: integerDefault: -1

### CC_RAS3_MAX¶

maximum number of electrons in RAS III, for CC

Type: integerDefault: -1

### CC_RAS4_MAX¶

maximum number of electrons in RAS IV, for CC

Type: integerDefault: -1

### CC_VAL_EX_LEVEL¶

The CC valence excitation level

Type: integerDefault: 0

### CC_VECS_READ¶

Do import a CC vector from disk?

Type: booleanDefault: false

### CC_VECS_WRITE¶

Do export a CC vector to disk?

Type: booleanDefault: false

### DIIS_FREQ¶

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

Type: integerDefault: 1

### DIIS_MAX_VECS¶

Maximum number of error vectors stored for DIIS extrapolation

Type: integerDefault: 5

### DIIS_MIN_VECS¶

Minimum number of error vectors stored for DIIS extrapolation

Type: integerDefault: 2

### DIIS_START_ITER¶

Iteration at which to start using DIIS

Type: integerDefault: 1

### NUM_AMPS_PRINT¶

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

Type: integerDefault: 10

## MCSCF¶

### DF_BASIS_MCSCF¶

Auxiliary basis set for MCSCF density fitted ERI computations. This only effects the “Q” matrix in Helgaker’s language. Defaults to a JKFIT basis.

Type: stringPossible Values: basis stringDefault: No Default

### MCSCF_ALGORITHM¶

Convergence algorithm to utilize. Two-Step, Augmented Hessian, or One-Step. Defaults to TS for RASSCF.

Type: stringPossible Values: TS, AHDefault: TS

### MCSCF_DIIS_ERROR_TYPE¶

DIIS error vector type either, the AO orbital gradient or the orbital rotation update matrix

Type: stringPossible Values: GRAD, UPDATEDefault: GRAD

### MCSCF_DIIS_FREQ¶

How often to do a DIIS extrapolation for TS convergence

Type: integerDefault: 1

### MCSCF_DIIS_MAX_VECS¶

Maximum number of DIIS vectors for TS convergence

Type: integerDefault: 8

### MCSCF_DIIS_START¶

Iteration to turn on DIIS for TS convergence

Type: integerDefault: 3

### MCSCF_E_CONVERGENCE¶

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

Type: conv doubleDefault: 1e-7

### MCSCF_GUESS¶

Initial MCSCF starting guess, MP2 natural orbitals only available for DF-RHF reference

Type: stringPossible Values: MP2, SCFDefault: SCF

### MCSCF_MAXITER¶

Maximum number MCSCF of iterations

Type: integerDefault: 30

### MCSCF_MAX_ROT¶

Maximum value in the rotation matrix. If a value is greater than this number all values are scaled.

Type: doubleDefault: 0.5

### MCSCF_ROTATE¶

Apply a list of 2x2 rotation matrices to the orbitals in the form of [irrep, orbital1, orbital2, theta] where an angle of 0 would do nothing and an angle of 90 would switch the two orbitals.

Type: arrayDefault: No Default

### MCSCF_R_CONVERGENCE¶

Convergence criterion for the RMS of the orbital gradient

Type: conv doubleDefault: 1e-5

### MCSCF_SO_START_E¶

Start second-order (AH or OS) orbital-orbital MCSCF based on energy convergence

Type: doubleDefault: 1e-4

### MCSCF_SO_START_GRAD¶

Start second-order (AH or OS) orbital-orbital MCSCF based on RMS of orbital gradient

Type: doubleDefault: 1e-4

### MCSCF_TYPE¶

Method to handle the two-electron integrals

Type: stringPossible Values: DF, CONV, AODefault: CONV

*Expert* General Options¶

### CI_NUM_THREADS¶

Number of threads for DETCI.

Type: integerDefault: 1

### EX_ALLOW¶

An array of length EX_LEVEL specifying 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

### R4S¶

Do restrict strings with \(e-\) 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

### SF_RESTRICT¶

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

### SIGMA_OVERLAP¶

Do print the sigma overlap matrix? Not generally useful.

Type: booleanDefault: false

*Expert* Diagonalization Methods¶

### H0_BLOCKSIZE¶

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_VECTOR is

`H0_BLOCK`

. Defaults to 1000. Note that the program may change the given size for Ms=0 cases MS0 is 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: 1000

### H0_BLOCK_COUPLING¶

Do use coupling block in preconditioner?

Type: booleanDefault: false

### H0_BLOCK_COUPLING_SIZE¶

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

Type: integerDefault: 0

### H0_GUESS_SIZE¶

size of H0 block for initial guess

Type: integerDefault: 1000

### HD_AVG¶

How to average H diag energies over spin coupling sets.

`HD_EXACT`

uses the exact diagonal energies which results in expansion vectors which break spin symmetry.`HD_KAVE`

averages the diagonal energies over a spin-coupling set yielding spin pure expansion vectors.`ORB_ENER`

employs the sum of orbital energy approximation giving spin pure expansion vectors but usually doubles the number of Davidson iterations.`EVANGELISTI`

uses the sums and differences of orbital energies with the SCF reference energy to produce spin pure expansion vectors.`LEININGER`

approximation 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 as`EVANGELISTI`

.

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

*Expert* Density Matrices¶

*Expert* Root Following¶

### FOLLOW_VECTOR¶

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

*Expert* Guess Vectors¶

### CI_FILE_START¶

What file do we start at for hd/c/s/d CIvects? Should be 50 for normal CI calculations and 54 if we are going to do a second monomer.

Type: integerDefault: 50

### FILTER_GUESS¶

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_DET1 FILTER_GUESS_DET2 and FILTER_GUESS_SIGN

Type: booleanDefault: false

### FILTER_GUESS_DET1¶

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

### FILTER_GUESS_DET2¶

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

### FILTER_GUESS_SIGN¶

The required phase (1 or -1) between the two determinants specified by FILTER_GUESS_DET1 and FILTER_GUESS_DET2

Type: integerDefault: 1

### FILTER_ZERO_DET¶

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¶

Guess vector type. Accepted values are

`UNIT`

for a unit vector guess NUM_ROOTS and NUM_INIT_VECS must both be 1);`H0_BLOCK`

to use eigenvectors from the H0 BLOCK submatrix (default);`DFILE`

to use NUM_ROOTS previously converged vectors in the D file;

Type: stringPossible Values: UNIT, H0_BLOCK, DFILEDefault: H0_BLOCK

### NUM_INIT_VECS¶

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

Type: integerDefault: 0

### REFERENCE_SYM¶

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

*Expert* File Handling¶

*Expert* General-Order Perturbation Theory¶

### MPN_ORDER_SAVE¶

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 from MPN_WIGNER = true).

Type: integerDefault: 0

### MPN_SCHMIDT¶

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

Type: booleanDefault: false

### MPN_WIGNER¶

Do use Wigner formulas in the \(E_{text{mp}n}\) series?

Type: booleanDefault: true

### PERTURB_MAGNITUDE¶

The magnitude of perturbation \(z\) in \(H = H_0 + z H_1\)

Type: doubleDefault: 1.0

*Expert* General-Order Coupled-Cluster¶

### CC_FIX_EXTERNAL¶

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

Type: booleanDefault: false

### CC_FIX_EXTERNAL_MIN¶

Number of external indices before amplitude gets fixed by CC_FIX_EXTERNAL Experimental.

Type: integerDefault: 1

### CC_MACRO¶

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

### CC_MIXED¶

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

### CC_UPDATE_EPS¶

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

Type: booleanDefault: true

### CC_VARIATIONAL¶

Do use variational energy expression in CC computation? Experimental.

Type: booleanDefault: false

*Expert* Alternative Algorithms¶

### BENDAZZOLI¶

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

### FCI_STRINGS¶

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

Type: booleanDefault: false

### REPL_OTF¶

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