detcas(1)

📦 version 3.4.0, in debian bookworm, ubuntu jammy, ubuntu noble

💅 detcas — bestie, it's the CASSCF era

okay so hi, welcome to detcas, aka the Determinant Configuration Interaction / Complete Active Space Self-Consistent-Field Program. that's the whole name, it's a mouthful, we say it with our chest anyway. ✨

🌸 NAME 🌸

detcas — Determinant Configuration Interaction / Complete Active Space Self-Consistent-Field Program. she does orbitals. she does them self-consistently. periodt.

🦋 DESCRIPTION 🦋

so like, detcas does a complete-active-space self-consistent-field (CASSCF) optimization of molecular orbitals, and she does it via a two-step procedure working together with the determinant configuration interaction program DETCI. iconic duo, no cap.

the program is fairly simple (her words, not mine) and currently uses a Newton-Raphson approach to update the orbitals, employing a simple approximate orbital Hessian. convergence gets accelerated using Pulay's direct inversion of the iterative subspace (DIIS) procedure. 💫

the code has been written to allow more general wavefunctions that don't necessarily feature a full CI treatment of the active space. in particular, any restricted active space (RAS) CI wavefunction supported by DETCI can be used, which means you can get MCSCF wavefunctions of the RASSCF type. we love range. 💅

📚 REFERENCES 📚

Approximate Orbital Hessian:

  1. G. Chaban, M. W. Schmidt, and M. S. Gordon, Theor. Chim. Acta 97, 88-95 (1997).

Restricted Active Space CI:

  1. Determinant Based Configuration Interaction Algorithms for Complete and Restricted Configuration Interaction Spaces, J. Olsen, B. O. Roos, P. Jorgensen, and H. J. Aa. Jensen, J. Chem. Phys. 89, 2185 (1988).

Restricted Active Space SCF:

  1. P.-A. Malmqvist, A. Rendell, and B. O. Roos, J. Phys. Chem. 94, 5477 (1990).

DETCI Program:

  1. C. D. Sherrill, Computational Algorithms for Large-Scale Full and Multi-Reference Configuration Interaction Wavefunctions, PhD thesis, University of Georgia, Athens, GA, 1996.
  2. C. D. Sherrill and H. F. Schaefer, The Configuration Interaction Method: Advances in Highly Correlated Approaches, Adv. Quantum Chem. 34, 143-269 (1999).

like she CITED her sources, she's an academic baddie, we stan. 🎀

🌈 FILES REQUIRED 🌈

these ones she needs, bestie. no negotiating:

input.dat          - Input file
file78             - MO one-electron integrals (fzc operator)
file72             - MO two-electron integrals
file73             - MO one-particle density matrix
file74             - MO two-particle density matrix
file75             - MO Lagrangian

✨ FILES UPDATED ✨

and these ones she writes to, so watch out:

output.dat         - Output file
file14.dat         - Record of energies and orbital gradients

🎀 INPUT FORMAT 🎀

The following command-line arguments are available:

  • -quiet — same result as PRINT=0, she's just being quiet about it. 🤫
  • -o fname — gives the filename for the output file. defaults to output.dat.

Additional input for this program is read from the file input.dat. the more commonly used keywords are:

💖 the ones you'll actually use 💖

  • WFN = string — the supported wave function types are CASSCF and RASSCF. pick your fighter.
  • CONVERGENCE = integer — convergence desired on the orbital gradient. convergence is achieved when the RMS of the error in the orbital gradient is less than 10**(-n). the default is 4 for energy calculations and 7 for gradients.
  • DOCC = integer_array — this vector gives the number of doubly occupied orbitals in each irrep. there is no default. ngl you gotta bring your own.
  • SOCC = integer_array — this vector gives the number of singly occupied orbitals in each irrep. there is no default. same energy.
  • ENERGY_CONVERGENCE = integer — convergence desired on the total MCSCF energy. the default is 7.
  • FROZEN_DOCC = integer_array — the number of lowest energy doubly occupied orbitals in each irreducible representation which will literally be frozen (not updated in the MCSCF). the Cotton ordering of the irredicible representations is used. the default is the zero vector.
  • FROZEN_UOCC = integer_vector — the number of highest energy unoccupied orbitals in each irreducible representation which will literally be frozen (not updated in the MCSCF). the default is the zero vector.
  • RESTRICTED_DOCC = integer_array — the number of lowest energy doubly occupied orbitals in each irreducible representation which will be optimized but kept doubly occupied in the MCSCF. these orbitals come after the FROZEN_DOCC orbitals. the default is the zero vector.
  • RESTRICTED_UOCC = integer_array — the number of highest energy unoccupied orbitals in each irreducible representation which will be optimized but kept unoccupied in the MCSCF. these orbitals come before the FROZEN_UOCC orbitals. the default is the zero vector.
  • NCASITER = integer — maximum number of iterations to optimize the orbitals. > this option should be specified in the DEFAULT section of input, because it needs to be visible to the control program PSI. defaults to 1.
  • PRINT = integer — this option determines the verbosity of the output. a value of 1 or 2 specifies minimal printing, a value of 3 specifies verbose printing. values of 4 or 5 are used for debugging. do not use level 5 unless the test case is very small (e.g. STO H2O CISD). 💀 she said be careful.

🧚 the less commonly used keywords 🧚

these are the deep cuts, the b-sides, the iykyk. 🌸

  • DIIS_FREQ = integer — the DIIS extrapolation procedure will be attempted every n iterations. the default is 1.
  • DIIS_MAX_VECS = integer — the maximum number of subspace vectors for the DIIS procedure. after this number of vectors are reached, older vectors will be dropped from the subspace as necessary. the default is 8.
  • DIIS_MIN_VECS = integer — the minimum number of subspace vectors before a DIIS interpolation can be performed. the default is 2. it doesn't make sense to have values less than 2. (fr fr, don't.)
  • DIIS_START = integer — the iteration number when Pulay's Direct Inversion of the Iterative Subspace (DIIS) procedure for acceleration of convergence should be turned on. the default is 3. prior to this iteration, vectors are not added to the DIIS subspace. the first DIIS step will not be taken until DIIS_MIN_VECS vectors are in the DIIS subspace.
  • LAG_FILE = integer — file (unit number) for reading the lagrangian matrix. the default value is currently 75.
  • OPDM_FILE = integer — file (unit number) for reading the one-particle density matrix. the default value is currently 73.
  • SCALE_GRAD = boolean — tells whether to scale the orbital gradient by the approximate (diagonal) orbital Hessian. the default is TRUE.
  • SCALE_STEP = real — scale factor for the orbital rotation step. default is 1.0.
  • TPDM_FILE = integer — file (unit number) for reading the two-particle density matrix. the default value is currently 74.
  • LEVEL_SHIFT = boolean — tells whether to allow for level shifting of the hessian matrix. this can be used to ensure that the hessian is positive definite for the beginning iterations. > if the hessian is not positive definite near convergence, then this may be a sign of numerical instabilities in the MCSCF. the default is TRUE.
  • SHIFT = real — tells how much the diagonal elements of the hessian should be shifted for level shifting. default is 0.01.
  • DETERM_MIN = real — minimum allowed valued for the determinant of the hessian matrix if level shifting is on. this is not used if LEVEL_SHIFT=FALSE. default is 0.00001.
  • STEP_MAX = real — this is the maximum allowed single orbital rotation. default is 0.30.
  • FORCE_STEP = boolean — this allows the user to overide the calculated step and to force a step in a particular direction. this can be useful if trying to force the calculation away from a saddle point. the default is FALSE.
  • FORCE_PAIR = integer — this is the index for the independent pair which is to be rotated if FORCE_STEP=TRUE. this is ignored otherwise. there is no default.
  • FORCE_VALUE = real — this is the orbital rotation value for rotating the orbitals specified by FORCE_PAIR. this is only used if FORCE_STEP is set to TRUE. the default value is 0.0.
  • CHECK_HESSIAN = boolean — calculate the eigenvalues of the orbital hessian. this is good for checking for saddle-point solutions. should be used without level shifting. the default is FALSE.
  • EIGEN_VECTORS = boolean — do you want to calculate the eigenvectors of the orbital hessian as well. this is ignored if CHECK_HESSIAN=FALSE. default is FALSE.

and that's the whole page, bestie!! go optimize some orbitals, you're literally the main character of quantum chemistry 💖✨

this was detcas(1), brought to you by the good people at source/version detcas(1), dated 8 May, 1998 — yeah she's a vintage icon, an elder, and she still ate. 🫶👑

📦 where this page comes from