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24.1 Coupled-cluster, CCSD

The command CCSD performs a closed-shell coupled-cluster calculation. Using the CCSD(T) command, the perturbative contributions of connected triple excitations are also computed.

If the CCSD is not converged, an error exit will occur if triples are requested. This can be avoided using the NOCHECK option:

CCSD(T),NOCHECK

In this case the (T) correction will be computed even if the CCSD did not converge. Note: NOCHECK has no effect in geometry optimizations or frequency calculations.

For further information on triples corrections see under RCCSD.

In cases that the Hartree-Fock reference determinant does not sufficiently dominate the wavefunction, the program will stop with a message UNREASONABLE NORM. CALCULATION STOPPED. This can be avoided by setting the option CC_NORM_MAX=value on the CCSD command line. The calculation is stopped if the square norm of the wavefunction (in intermediate normalization) exceeds 1+nval*CC_NORM_MAX, where nval is the number of correlated valence orbitals. The default value is CC_NORM_MAX=0.2.



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