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Important options:

The options to define the orbital and the auxiliary basis sets, the maximum amount allocatable core memory ($maxcor), and the frozen-core approximation ($maxcor) have been mentioned above and described in the previous chapters on MP2 and CC2 calculations. Apart from this, CCSD and CCSD(T) calculations require very little additional input.

Relevant are in particular some options in the $ricc2 data group:

$ricc2
  ccsd
  ccsd(t)
  conv=7
  oconv=6
  mxdiis=10
  maxiter=25
The options ccsd and ccsd(t) request, respectively, CCSD and CCSD(T) calculations. Since CCSD(T) requires the cluster amplitudes from a converged CCSD calculation, the option ccsd(t) is implies the ccsd option.

The number given for mxdiis defines the maximum number of vectors included in the DIIS procedure for the solution of the cluster equations. As mentioned above, it has some impact on the amount of disc space used by a CCSD calculation. Unless disc space becomes a bottleneck, it is not recommended to change the default value.

With maxiter one defines the maximum number of iterations for the solution of the cluster equations. If convergence is not reached within this limit, the calculation is stopped. Usually 25 iterations should be sufficient for convergence. Only in difficult cases with strong correlation effects more iterations are needed. It is recommended to increase this limit only if the reason for the strong correlation effects is known. (Since one reason could also be an input error as e.g. unreasonable geometries or orbital occupations as a wrong basis set assignment.)

The two parameters conv and oconv define the convergence thresholds for the iterative solution of the cluster equations. Convergence is assumed if the change in the energy (with respect to the previous iteration) has is smaller than 10-conv and the euclidian norm of the residual (the so-called vector function) is smaller than 10-oconv. If conv is not given in the data group $ricc2 the threshold for changes in the energy is set to value given in $denconv (by default 10-7). If oconv is not given in the data group $ricc2 the threshold for the residual norm is by default set to 10 times the threshold changes in the energy. With the default settings for these thresholds, the energy will thus be converged until changes drop below 10-7 Hartree, which typically ensures an accuracy of about μH. These setting are thus rather tight and conservative even for the calculation of highly accurate reaction energies. If for your application larger uncertainites for the energy are tolerable, it is recommended to use less tight thresholds, e.g. conv=6 or conv=5 for an accuracy of, respectively, at least 0.01 mH (0.03 kJ/mol) or 0.1 mH (0.3 kJ/mol). The settings for conv (and oconv) have not only an impact on the number of iterations for the solution of the cluster equations, but as they determine the thresholds for integral screening also to some extend on the costs for the individual iterations.


next up previous contents index
Next: Calculation of Vibrational Frequencies Up: Characteristics of the Implementation Previous: Memory requirements:   Contents   Index
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