ccenergy(1)

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

💖 ccenergy — bestie edition 💖

okay so you found the ccenergy(1) page, the manual for the Coupled cluster singles and doubles energy program. sit down, grab a strawberry, we're doing this together 🍓✨

🌸 NAME 🌸

ccenergy — the coupled cluster singles and doubles energy program. that's her. that's the whole vibe. 👑

✨ DESCRIPTION ✨

okay so ccenergy solves the coupled cluster singles and doubles amplitude equations. it's giving quantum chemistry main character energy fr fr.

she works for spin-restricted closed-shell Hartree-Fock (RHF), spin-restricted open-shell Hartree-Fock (ROHF), spin-unrestricted Hartree-Fock (UHF), and Brueckner references. so many references, so little time 💅

the algorithm uses spatial symmetry (that's D2h and all its subgroups, iykyk) plus matrix algebra techniques for efficiency, because she's not doing all that work for nothing.

and bestie, bonus: the program can also be used to simulate local correlation methods. versatile queen 🌈

📚 REFERENCES 📚

  1. An introduction to coupled cluster theory for computational chemists, T.D. Crawford and H.F. Schaefer, Rev. Comp. Chem. 14 33-136 (2000).
  2. A direct product decomposition approach for symmetry exploitation in many-body methods. I. Energy calculations, J.F. Stanton, J. Gauss, J.D. Watts, and R.J. Bartlett, J. Chem. Phys. 94 4334 (1991).

like, cite your sources bestie, it's the bare minimum 🫶

📝 INPUT FORMAT 📝

input for this program is read from the file input.dat. no cap, that's the file. these are the keywords that are valid:

  • WFN = string — specifies the type of coupled cluster calculation you want, your dream CC vibe. valid entries at present are CCSD, CCSD_T, BCCD, and BCCD_T. there is no default, so you literally have to pick one, okay? 🙈
  • REFERENCE = string — specifies the type of orbitals used for the single-determinant reference function. valid entries at present are RHF, UHF, and ROHF. again, there is no default. choose your fighter ⭐
  • MEMORY = (real MB) — the maximum amount of core memory to be used, in megabytes. see also the CACHETYPE and CACHELEV keywords below, they're besties.
  • CONVERGENCE = integer — the convergence desired on the CC amplitudes. convergence is achieved when the RMS of the error in the CC wave function is less than 10**(-n). the default is 7. slay, precision 💅
  • BRUECKNER_CONV = integer — the convergence desired for the orbitals in Brueckner-CC calculations. convergence is achieved when the largest absolute value of a single T1 amplitude is less than 10**(-n). the default is 5.
  • MAXITER = integer — maximum number of iterations to solve the coupled cluster equations. defaults to 50. fifty chances, make them count 🔥
  • FREEZE_CORE = string — specifies whether core orbitals should be automatically frozen in the CC wave function. if this option is set (preferably in the "default" section of input), then the cscf program will identify the core orbitals. subsequent codes starting with transqt will freeze these orbitals. note that this keyword usually makes the additional specification of FROZEN_DOCC unnecessary, but it may still be required in some special cases. allowed values are true, false, yes, no, 1, 0, large and small.
  • FROZEN_DOCC = integer_array — the number of lowest energy doubly occupied orbitals in each irreducible representation from which there will be no excitations. the Cotton ordering of the irreducible representations is used. the default is the zero vector. NB: this keyword is actually read by the integral transformation program transqt and the integral sorting program ccsort. see also the FREEZE_CORE keyword. sneaky little handoff, but okay 👀
  • FROZEN_UOCC = integer_vector — the number of highest energy unoccupied orbitals in each irreducible representation into which there will be no excitations. the default is the zero vector. NB: this keyword is actually read by the integral transformation program transqt and the integral sorting program ccsort.
  • PRINT = integer — determines the verbosity of the output. a value of 0 (the default) specifies minimal printing, and a value of 2 will give output updates as each term of the CC equations is complete. no other values have yet been used. so like, it's 0 or 2, that's the menu 🎀
  • NUM_AMPS = integer — specifies the number of largest T1 and T2 amplitudes to print at the end of the CC procedure. default is 10.
  • PRINT_MP2_AMPS = boolean — indicates if the MP2 amplitudes (which are starting guesses for RHF and UHF reference functions) should be printed. default is false.
  • RESTART = boolean — allows the program to use old T1 and T2 vectors from previous calculations as the initial guesses for a new calculation. this is particularly useful for geometry optimizations. the default is TRUE. NB: the ability to restart a calculation is also controlled by the phases of the molecular orbitals. if the parameter PHASE from the chkpt file (cf. libchkpt) is set to FALSE, then the user's RESTART input will be ignored. this behavior can be overridden, however, with the FORCE_RESTART option. the drama of it all 😭
  • FORCE_RESTART = boolean — forces possible restart of the CC equations from previous T1 and T2 amplitudes.
🚨 bestie listen: this option should be used only by experts. like, actual experts. not "I read one paper" experts.
  • AO_BASIS = string — compute contributions from four virtual-index integrals, <ab|cd>, from the atomic-orbital basis integrals. this option slows the calculation somewhat, but greatly reduces disk-space requirements. allowed values are NONE (default), DISK (use the AO-basis integrals stored on disk), or DIRECT (use cints to compute the AO-integrals on the fly in each CC iteration).
the DIRECT option is still experimental and should be used only by experts. do not freestyle with it, I'm begging 🙏
  • CACHETYPE = string — selects the priority type for maintaining the automatic memory cache used by the DPD codes. (see libdpd.html for further details.) a value of LOW (the default) selects a "low priority" scheme in which the deletion of items from the cache is based on pre-programmed priorities. a value of LRU selects a "least recently used" scheme in which the oldest item in the cache will be the first one deleted. it's giving cache eviction policy discourse 💀
  • CACHELEV = integer — selects the level of automatic caching desired in the storage of various amplitudes, integrals, and intermediates in the coupled cluster procedure. a value of 0 retains no quantities in cache, while a level of 6 attempts to store all quantities in cache. for particularly large calculations, a value of 0 may help with certain types of memory problems. the default is 2, which means that all four-index quantities with up to two virtual-orbital indices (e.g., <ij|ab> integrals) may be held in the cache.
  • DIIS = boolean — selects the use of the direct-inversion in the iterative subspace (DIIS) technique of Pulay for convergence acceleration. the default is TRUE. thanks Pulay, icon 🫶
  • LOCAL = boolean — simulate the effects of local correlation techniques. the default is FALSE.
  • LOCAL_CUTOFF = real — the tolerance value (always between one and zero) for the Broughton-Pulay completeness check used to construct orbital domains for local-CC calculations. the default is 0.02. see J. Broughton and P. Pulay, J. Comp. Chem. 14, 736-740 (1993) and C. Hampel and H.-J. Werner, J. Chem. Phys. 104, 6286-6297 (1996).
  • LOCAL_METHOD = string — selects the type of local-CCSD scheme to be simulated. valid entries are WERNER for the method developed by H.-J. Werner and co-workers, or AOBASIS for the method developed by G.E. Scuseria and co-workers (currently inoperative). the default is WERNER.
  • LOCAL_WEAKP = string — selects the desired treatment of "weak pairs" in the local-CCSD method. a value of NEGLECT ignores weak pairs entirely. a value of NONE treats weak pairs in the same manner as strong pairs. a value of MP2 (the default) uses second-order perturbation theory to correct the local-CCSD energy computed with weak pairs ignored. weak pairs deserve rights too ✨
  • PRINT_PAIR_ENERGIES = boolean — indicates if the MP2 and CCSD pair energies should be printed. this is only possible for RHF references. default is false.
  • SPINADAPT_ENERGIES = boolean — indicates if spin-adapted pair energies should be printed. default is false.

and that's the whole page, bestie. you ate. 💅

this lil' manual is for ccenergy and it's dated 18 June, 2002 — vintage, lowkey iconic, and still serving. go run some coupled cluster, you've got this 🦋💜

periodt. ✨

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