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:

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`.

molpro@molpro.net 2019-05-24