Provided by: psi3_3.4.0-6_amd64

**NAME**

ccenergy - Coupled cluster singles and doubles energy program

**DESCRIPTION**

The programccenergysolves the coupled cluster singles and doubles amplitude equations. This code will work for spin-restricted closed-shell Hartree-Fock (RHF), spin-restricted open-shell Hartree-Fock (ROHF), spin-unrestricted Hartree-Fock (UHF), and Brueckner references. The algorithm makes use of spatial symmetry (D2h and its subgroups) and matrix algebra techniques for efficiency. In addition, the program can be used to simulate local correlation methods.

**REFERENCES**

1. An introduction to coupled cluster theory for computational chemists, T.D. Crawford and H.F. Schaefer, Rev. Comp. Chem.1433-136 (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.944334 (1991).

**INPUT** **FORMAT**

Input for this program is read from the file input.dat. The following keywords are valid:WFN=stringSpecifies the type of coupled cluster calculation desired. Valid entries at present areCCSD,CCSD_T,BCCD, andBCCD_T. There is no default.REFERENCE=stringSpecifies the type of orbitals used for the single-determinant reference function. Valied entries at present areRHF,UHF, andROHF. There is no default.MEMORY=(realMB) The maximum amount of core memory to be used, in megabytes. See also theCACHETYPEandCACHELEVkeywords below.CONVERGENCE=integerConvergence 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.BRUECKNER_CONV=integerConvergence desired for the orbitals in Brueckner-CC calculations. Convergence is achieved when the largest absolute value of a single T1 ampltiudes is is less than 10**(-n). The default is 5.MAXITER=integerMaximum number of iterations to solve the coupled cluster equations. Defaults to 50.FREEZE_CORE=stringSpecifes 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 thecscfprogram will identify the core orbitals. Subsequent codes starting withtransqtwill 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_arrayThe number of lowest energy doubly occupied orbitals in each irreducible representation from which there will be no excitations. The Cotton ordering of the irredicible representations is used. The default is the zero vector.NB:This keyword is actually read by the integral transformation programtransqtand the integral sorting programccsort.See also the FREEZE_CORE keyword.FROZEN_UOCC=integer_vectorThe 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 programtransqtand the integral sorting programccsort.=integerDetermines 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.NUM_AMPS=integerSpecifies the number of largest T1 and T2 amplitudes to print at the end of the CC procedure. Default is 10.PRINT_MP2_AMPS=booleanIndicates if the MP2 amplitudes (which are starting guesses for RHF and UHF reference functions) should be printed. Default is false.RESTART=booleanAllows 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 isTRUE.NB:The ability to restart a calculation is also controlled by the phases of the molecular orbitals. If the parameterPHASEfrom the chkpt file (cf. libchkpt) is set toFALSE, then the user'sRESTARTinput will be ignored. This behavior can be overridden, however with theFORCE_RESTARToption.FORCE_RESTART=booleanForces possible restart of the CC equations from previous T1 and T2 amplitudes. This option should be used only by experts.AO_BASIS=stringCompute 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 (usecintsto compute the AO- integrals on the fly in each CC iteration). The DIRECT option is still experimental and should be used only by experts.CACHETYPE=stringSelects the priority type for maintaining the automatic memory cache used by the DPD codes. (See libdpd.html for further details.) A value ofLOW(the default) selects a "low priority" scheme in which the deletion of items from the cache is based on pre-programmed priorities. A value ofLRUselects a "least recently used" scheme in which the oldest item in the cache will be the first one deleted.CACHELEV=integerSelects the level of automatic cacheing 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 quantites with up to two virtual-orbital indices (e.g., <ij|ab> integrals) may be held in the cache.DIIS=booleanSelects the use of the direct-inversion in the iterative subspace (DIIS) technique of Pulay for convergence acceleration. The default isTRUE.LOCAL=booleanSimluate the effects of local correlation techniques. The default isFALSE.LOCAL_CUTOFF=realThe tolerance value (always between one and zero) for the Broughton-Pulay completeness check used to contruct 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=stringSelects the type of local-CCSD scheme to be simulated. Valid entries areWERNERfor the method develop by H.-J. Werner and co-workers orAOBASISfor the method developed by G.E. Scuseria and co-workers (currently inoperative). The default isWERNER.LOCAL_WEAKP=stringSelects the desired treatment of "weak pairs" in the local-CCSD method. A value ofNEGLECTignores weak pairs entirely. A value ofNONEtreats weak pairs in the same manner as strong pairs. A value ofMP2(the default) uses second-order perturbation theory to correct the local-CCSD energy computed with weak pairs ignored.PRINT_PAIR_ENERGIES=booleanIndicates if the MP2 and CCSD pair energies should be printed. This is only possible for RHF references. Default is false.SPINADAPT_ENERGIES=booleanIndicates if spin-adapted pair energies should be printed. Default is false. 18 June, 2002 ccenergy(1)