It is important to ensure there are sufficient resources available for the necessary integral calculations and transformations. For CCMAN/CCMAN2 algorithms, these resources are controlled using the $rem variables CC_MEMORY, MEM_STATIC and MEM_TOTAL (see Section 6.14).
The exact flavor of correlation treatment within equation-of-motion methods is defined by METHOD (see Section 7.1). For EOM-CCSD, once should set METHOD to EOM-CCSD, for EOM-MP2, METHOD = EOM-CCSD, etc.. In addition, a specification of the number of target states is required through XX_STATES (XX designates the type of the target states, e.g., EE, SF, IP, EA, DIP, DSF, etc.). Users must be aware of the point group symmetry of the system being studied and also the symmetry of the initial and target states of interest, as well as symmetry of transition. It is possible to turn off the use of symmetry by CC_SYMMETRY. If set to FALSE the molecule will be treated as having symmetry and all states will be of symmetry.
Note: 1. In finite-difference calculations, the symmetry is turned off automatically, and the user must ensure that XX_STATES is adjusted accordingly. 2. In CCMAN, mixing different EOM models in a single calculation is only allowed in Dyson orbitals calculations. In CCMAN2, different types of target states can be requested in a single calculation.
Below we describe alternative way to specify correlation treatment in EOM-CC/CI calculations. These keywords will be eventually phased out. By default, the level of correlation of the EOM part of the wave function (i.e., maximum excitation level in the EOM operators ) is set to match CORRELATION, however, one can mix different correlation levels for the reference and EOM states by using EOM_CORR. To request a CI calculation, set CORRELATION = CI and select type of CI expansion by EOM_CORR. The table below shows default and allowed CORRELATION and EOM_CORR combinations.
|CORRELATION||Default||Allowed||Target states||CCMAN /|
|CI||none||CIS, CIS(D)||EE, SF||y/n|
|CISD||EE, SF, IP||y/n|
|SDT, DT||EE, SF, DSF||y/n|
|CCSD, OD||CISD||EE, SF, IP, EA, DIP||y/y|
|SD(fT)||EE, IP, EA||n/y|
|SD(dT), SD(fT)||EE, SF, fake IP/EA||y/n|
|SD(dT), SD(fT), SD(sT)||IP||y/n|
|SDT, DT||EE, SF, IP, EA, DIP, DSF||y/n|
Table 7.2 shows the correct combinations of CORRELATION and EOM_CORR for standard EOM and CI models.
|Method||CORRELATION||EOM_CORR||Target states selection|
|SF-CISDT||CI||SDT or DT||SF_STATES|
|EOM-2SF-CCSD||CCSD||SDT or DT||DSF_STATES|
The most relevant EOM-CC input options follow.
Note: It is a symmetry of a transition rather than that of a target state which is specified in excited state calculations. The symmetry of the target state is a product of the symmetry of the reference state and the transition. For closed-shell molecules, the former is fully symmetric and the symmetry of the target state is the same as that of transition, however, for open-shell references this is not so.
Note: For the XX_STATES options, Q-Chem will increase the number of roots if it suspects degeneracy, or change it to a smaller value, if it cannot generate enough guess vectors to start the calculations.
Note: When EOM_FAKE_IPEA is set to TRUE, it can change the convergence of Hartree-Fock iterations compared to the same job without EOM_FAKE_IPEA, because a very diffuse basis function is added to a center of symmetry before the Hartree-Fock iterations start. For the same reason, BASIS2 keyword is incompatible with EOM_FAKE_IPEA. In order to read Hartree-Fock guess from a previous job, you must specify EOM_FAKE_IPEA (even if you do not request for any correlation or excited states) in that previous job. Currently, the second moments of electron density and Mulliken charges and spin densities are incorrect for the EOM-IP/EA-CCSD target states.
Note: In CCMAN, setting EOM_PRECONV_SINGLES = N would result in N Davidson iterations pre-converging singles.
Note: Not available for EOM-EE in CCMAN2.
Note: Not available in CCMAN2.
Note: Not available in CCMAN2.