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Additional Recommendations For Excited-state Treatments

Among the available schemes provided by CFOUR, the EOM-CCSD/CCSD_LR approach is probably the most useful.

The corresponding excited-state calculations can be performed as efficiently as the corresponding ground-state calculations and as for those calculations, AO algorithms (keyword ABCDTYPE=AOBASIS) and the more efficient CC module xecc (keyword CC_PROG=ECC) should be used. Nevertheless, it might be useful to give a few additional hints:

• excited-state calculations are no black-box calculations and a careful checking of the results (is the required state obtained in the calculation?) is warranted;
• specification of the initial guess via an %excite* section is preferred, as this allows a more focussed search on the excited state of interest, while when using the keyword ESTATE_SYM the corresponding initial guesses are generated in a CIS calculations;
• using a computational RHF reference allows just to compute singlet excitation energies, triplet excitation energies for closed-shell molecules require a computational UHF reference. In addition, the required triplet-excited states can be only obtained using a %excite* guess;
• within analytic gradient calculations for excited states, just one excited state can be treated at the time.

Concerning calculations using CC3, CCSDT-3, and CCSDT, it should be realized that all of these schemes involve an iterative treatment of triple excitations and thus are extremely costly. Such calculations are only recommended if highly accurate excitation energies are needed. Furthermore, in a CC3, CCSDT-3, and CCSDT calculation just one root can be determined in one calculations, and no analytic gradients and no transition moments are available (in the case of CCSDT, such calculations nevertheless are possible using the MRCC code by M. Kallay which has been interfaced to CFOUR (see Interface to MRCC)).

While CFOUR provides also lower-level treatments of excited states (CIS, CIS(D), and CC2), it should be noted that these schemes are not as efficiently implemented and that for the extensive use of these schemes, other quantum-chemical program packages are probably better suited.

Note also that the keyword ESTATE_PROP should be only used in conjunction with EOM-CCSD/CCSD_LR.