Q-Chem provides access to certain singlet excited states – namely, those well-described by a single-electron HOMO-LUMO transition – via restricted open-shell Kohn-Sham (ROKS) theory. In contrast to the MOM approach (see Section 6.4), which requires separate SCF calculations of the non-Aufbau and triplet energies, the ROKS approach attempts to combine the properties of both determinants at the level of the Fock matrix in one SCF calculation. ROKS thus presents as a single SCF loop, but the structure of the Fock matrix differs from the ground-state case. Note that this excited-state method is distinct from ROKS theory for open-shell ground states.

The implementation of ROKS excited states in Q-Chem largely follows the theoretical framework established by Filatov and Shaik[343] and is described in detail in Ref. [344]. Singlet excited state energies and gradients are available, enabling single-point, geometry optimization and molecular dynamics.

To perform an ROKS excited state calculation, simply set the keywords ROKS and UNRESTRICTED both to TRUE (the latter sounds as a contradiction to the method’s name, but yes, this is what you should do). An additional keyword ROKS_LEVEL_SHIFT is included to assist in cases of convergence difficulties with a standard level-shift technique. It is recommended to perform a preliminary ground-state calculation on the system first, and then use the ground-state orbitals to construct the initial guess using SCF_GUESS=READ.

ROKS

Controls whether ROKS calculation will be performed.

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

FALSE

ROKS is not performed.

TRUE

ROKS will be performed.

RECOMMENDATION:

Set to TRUE if ROKS calculation is desired. You should also set UNRESTRICTED=TRUE

ROKS_LEVEL_SHIFT

Introduce a level shift of N/100 Hartree to aid convergence.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0

No shift

N

level shift of N/100 Hartree.

RECOMMENDATION:

Use in cases of problematic convergence.

**Example 6.123** RO-PBE0/6-311+G* excited state gradient of formaldehyde, using the ground state orbitals as an initial guess.

$comment ROKS excited state gradient of formaldehyde Use orbitals from ground state for initial guess $end $rem exchange pbe0 basis 6-311+G* scf_convergence 9 sym_ignore true $end $molecule 0 1 H -0.940372 0.000000 1.268098 H 0.940372 0.000000 1.268098 C 0.000000 0.000000 0.682557 O 0.000000 0.000000 -0.518752 $end @@@ $molecule read $end $rem roks true unrestricted false exchange pbe0 basis 6-311+G* jobtype force scf_convergence 9 sym_ignore true scf_guess read $end