These method keywords request a coupled cluster [Bartlett78, Pople78] calculations, using double substitutions from the Hartree-Fock determinant for CCD [Pople78], or both single and double substitutions for CCSD [Cizek69, Purvis82, Scuseria88, Scuseria89]. CC and QCID are synonyms for CCD. RO may be combined with CCSD for a restricted open shell energy calculation [Watts93].
FC
All frozen core options are available with this keyword. See the discussion of the FC options for full information.
T
Include triple excitations non-iteratively [Pople87] (CCSD only). CCSD-T is a synonym for CCSD(T).
E4T
Used with the T option to request inclusion of triple excitations for both the complete MP4 and to form CCSD(T).
T1Diag
Computes the T1 diagnostic of T. J. Lee and coworkers [Lee89] (CCSD only).
Conver=N
Sets the convergence calculations to 10-N on the energy and 10-(N-2) on the wavefunction. The default is N=7 for single points and N=8 for gradients.
MaxCyc=N
Specifies the maximum number of cycles for CCSD calculations.
SaveAmplitudes
Saves the converged amplitudes in the checkpoint file for use in a subsequent calculation (e.g., using a larger basis set). Using this option results in a very large checkpoint file, but also may significantly speed up later calculations.
ReadAmplitudes
Reads the converged amplitudes from the checkpoint file (if present). Note that the new calculation can use a different basis set, method (if applicable), etc. than the original one.
Analytic energies and gradients for CCD and CCSD, numerical gradients for CCSD(T), and numerical frequencies for all methods. The restricted open-shell (RO) method is available for CCSD and CCSD(T) energy calculations.
MP4, Transformation, QCISD
The Coupled Cluster energy appears in the output as follows (following the final correlation iteration):
DE(CORR)= -.54979226D-01 E(CORR)= -.75019641794D+02 … CCSD(T)= -.75019717665D+02
The CCSD energy is labeled E(CORR), and the energy including the non-iterative triples contribution is given in the final line.
Last updated on: 10 May 2009